Liquid crystal display device

ABSTRACT

To suppress a malfunction of a circuit due to deterioration in a transistor. In a transistor which continuously outputs signals having certain levels (e.g., L-level signals) in a pixel or a circuit, the direction of current flowing through the transistor is changed (inverted). That is, by changing the level of voltage applied to a first terminal and a second terminal (terminals serving as a source and a drain) every given period, the source and the drain are switched every given period. Specifically, in a portion which successively outputs signals having certain levels (e.g., L-level signals) in a circuit including a transistor, L-level signals having a plurality of different potentials (L-level signals whose potentials are changed every given period) are used as the signals having certain levels.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to semiconductor devices. In particular,the present invention relates to semiconductor devices formed usingtransistors and an operating method thereof. Further, the presentinvention relates to display devices including semiconductor devices andelectronic devices including the display devices.

2. Description of the Related Art

In recent years, display devices such as liquid crystal display devicesand light-emitting devices have actively developed. In particular, atechnique for forming a pixel circuit and a driver circuit including ashift register circuit or the like (hereinafter referred to as aninternal circuit) over the same insulator by using transistors formedusing a non-single-crystal semiconductor has actively developed, becausethe technique greatly contributes to reduction in power consumption andcost, improvement in reliability, and decrease in frame. The internalcircuit formed over the insulator is connected to a controller IC or thelike provided outside the insulator (hereinafter referred to as anexternal circuit) through an FPC (flexible printed circuit) or the like,and its operation is controlled.

In addition, a shift register circuit including transistors formed usinga non-single-crystal semiconductor has been devised as the internalcircuit formed over the insulator (see Reference 1).

However, since an output terminal of the shift register circuit is in afloating state in a certain period, there has been a problem in thatnoise is easily generated in the output terminal and that the shiftregister circuit malfunctions due to noise generated in the outputterminal.

In order to solve the foregoing problem, a shift register circuit whoseoutput terminal does not enter into a floating state has been devised.For example, in Reference 2, a technique by which a shift registercircuit is operated by so-called static drive has been proposed. In thiscase, since an output terminal of the shift register circuit does notenter into a floating state, noise generated in the output terminal canbe reduced.

REFERENCE

Reference 1: PCT International Publication No. 95/31804

Reference 2: Japanese Published Patent Application No. 2004-078172

SUMMARY OF THE INVENTION

In the case of performing static drive as disclosed in Reference 2, anoperating period is divided into a selection period during whichselection signals are output and a non-selection period during whichnon-selection signals are output. Most of the operating period is thenon-selection period. In the case where a selection signal has a highpotential (a high-level signal), a non-selection signal (having a lowpotential (a low-level signal)) is supplied to an output terminalthrough a transistor in the non-selection period. That is, a transistorfor supplying a low potential to the output terminal is on in most ofthe operating period of a circuit.

It is known that a transistor formed using a non-single crystalsemiconductor deteriorates in accordance with a time during which thetransistor is on and a potential applied to the transistor. In the casewhere the transistor deteriorates, for example, there is a problem inthat a shift in threshold voltage, where the threshold voltage isshifted positively, occurs and that a malfunction of a circuit occurs.

In addition, unlike a pixel, an analog switch (e.g., a transfer gate),or the like, a direction through which current flows is fixed in adigital circuit such as a shift register circuit or a latch circuit inmany cases. That is, since a source and a drain of a transistor arefixed, an electric field concentrates on a drain side and the transistoreasily deteriorates.

In view of the foregoing problems, it is an object to suppressdeterioration in a transistor. Alternatively, it is an object tosuppress a malfunction of a circuit due to deterioration in atransistor. Alternatively, it is an object to improve reliability of acircuit including a transistor.

In order to suppress deterioration in a transistor, in a transistorwhich continuously outputs signals having certain levels (e.g., L-levelsignals (low-level signals)) in a pixel or a circuit, the direction ofcurrent flowing through the transistor is changed (inverted). That is,by changing the level of voltage applied to a first terminal and asecond terminal (terminals serving as a source and a drain) every givenperiod, the source and the drain are switched every given period.

Therefore, in a portion which continuously outputs signals havingcertain levels (e.g., L-level signals) in a circuit including atransistor, L-level signals having a plurality of different potentials(L-level signals whose potentials are changed every given period) areused as the signals having certain levels. For example, in the casewhere L-level signals are continuously output through a transistor,signals whose potentials are switched between first potentials V_(LH)and second potentials V_(LL) (V_(LH)>V_(LL)) every given period can beused as the I-level signals. That is, by using signals whose potentialsare changed as the L-level signals and changing the direction of currentflowing through the transistor (switching a source and a drain),concentration of an electric field on a source side or a drain side issuppressed. Thus, deterioration in the transistor is suppressed.

Note that any potential may be used as the first potential V_(LH) andthe second potential V_(LL) as long as it serves as an L-level signal inthe circuit. For example, in the case where the L-level signal is anon-selection signal in the circuit, the first potential and the secondpotential may be set such that they serve as non-selection signals.Alternatively, as the plurality of potentials, three or more potentialsmay be set.

Alternatively, in the case where H-level signals (high-level signals)are continuously output through the transistor, signals whose potentialsare switched between first potentials V_(HH) and second potentialsV_(HL) (V_(HH)>V_(HL)) every given period can be used as the H-levelsignals. Note that any potential may be used as the first potentialV_(HH) and the second potential V_(HL) as long as it serves as anH-level signal in the circuit. For example, in the case where theH-level signal is a selection signal in the circuit, the first potentialand the second potential may be set such that they serve as selectionsignals.

In addition, in an example of the disclosed invention, a firsttransistor, a second transistor, and a third transistor are provided.One of a source and a drain of the first transistor is electricallyconnected to a first wiring, and the other of the source and the drainof the first transistor is electrically connected to a third wiring. Oneof a source and a drain of the second transistor is electricallyconnected to a second wiring, and the other of the source and the drainof the second transistor is electrically connected to the third wiring.A gate of the third transistor is electrically connected to the thirdwiring. The third transistor is turned on by a selection signal suppliedto the third wiring and is turned off by a non-selection signal. Theselection signal is supplied from the first wiring to the third wiringin a period during which the first transistor is on. The non-selectionsignal is supplied from the second wiring to the third wiring in aperiod during which the second transistor is on. At least one of theselection signal and the non-selection signal is a signal whosepotential is changed every predetermined period.

In addition, in an example of the disclosed invention, a firsttransistor, a plurality of second transistors, and a third transistorare provided. One of a source and a drain of the first transistor iselectrically connected to a first wiring, and the other of the sourceand the drain of the first transistor is electrically connected to athird wiring. One of a source and a drain of each of the plurality ofsecond transistors is electrically connected to a second wiring, and theother of the source and the drain of each of the plurality of secondtransistors is electrically connected to the third wiring. The pluralityof second transistors are connected in parallel with each other. A gateof the third transistor is electrically connected to the third wiring.The third transistor is turned on by a selection signal supplied to thethird wiring and is turned off by a non-selection signal. The selectionsignal is supplied from the first wiring to the third wiring in a periodduring which the first transistor is on. The non-selection signal is asignal whose potential is changed every predetermined period and issupplied from the second wiring to the third wiring in a period duringwhich one of the plurality of second transistors is on.

In addition, in an example of the disclosed invention, a firsttransistor, a second transistor, a third transistor, and a fourthtransistor are provided. One of a source and a drain of the firsttransistor is electrically connected to a first wiring, and the other ofthe source and the drain of the first transistor is electricallyconnected to a third wiring. One of a source and a drain of the secondtransistor is electrically connected to a second wiring, and the otherof the source and the drain of the second transistor is electricallyconnected to the third wiring. One of a source and a drain of the fourthtransistor is electrically connected to a fourth wiring, and the otherof the source and the drain of the fourth transistor is electricallyconnected to the third wiring. A gate of the third transistor iselectrically connected to the third wiring. The third transistor isturned on by a selection signal supplied to the third wiring and isturned off by a non-selection signal. The selection signal is suppliedfrom the first wiring to the third wiring in a period during which thefirst transistor is on. The non-selection signal is supplied from thesecond wiring or the fourth wiring to the third wiring in a periodduring which the second transistor is on or a period during which thefourth transistor is on. Different potentials are applied to the secondwiring and the fourth wiring, and the potential applied to the secondwiring and the potential applied to the fourth wiring are switched.

In this specification, in the case of using a transistor as a switch,the polarity (conductivity type) of the transistor is not particularlylimited to a certain type because it operates just as a switch. However,a transistor having polarity with smaller off-state current ispreferably used when the amount of off-state current is to besuppressed. Examples of a transistor with smaller off-state current area transistor provided with an LDD region, a transistor with a multi-gatestructure, and the like. Further, an n-channel transistor is preferablyused when a potential of a source terminal of the transistor which isoperated as a switch is close to a potential of a low-potential-sidepower supply (e.g., Vss, GND, or 0 V). On the other hand, a p-channeltransistor is preferably used when the potential of the source terminalis close to a potential of a high-potential-side power supply (e.g.,Vdd). This is because the absolute value of gate-source voltage can beincreased when the potential of the source terminal of the n-channeltransistor is close to a potential of a low-potential-side power supplyand when the potential of the source terminal of the p-channeltransistor is close to a potential of a high-potential-side powersupply, so that the transistor can be more accurately operated as aswitch. This is also because the transistor does not often performsource follower operation, so that reduction in output voltage does notoften occur.

Note that a CMOS switch may be used as a switch by using both ann-channel transistor and a p-channel transistor. By using a CMOS switch,the switch can be more accurately operated as a switch because currentcan flow when either the p-channel transistor or the n-channeltransistor is turned on. For example, voltage can be appropriatelyoutput regardless of whether voltage of an input signal to the switch ishigh or low. In addition, since the voltage amplitude value of a signalfor turning on or off the switch can be made smaller, power consumptioncan be reduced.

Note that when a transistor is used as a switch, the switch includes aninput terminal (one of a source terminal and a drain terminal), anoutput terminal (the other of the source terminal and the drainterminal), and a terminal for controlling conduction (a gate terminal).On the other hand, when a diode is used as a switch, the switch does notinclude a terminal for controlling conduction in some cases. Therefore,when a diode is used as a switch, the number of wirings for controllingterminals can be further reduced as compared to the case of using atransistor.

Note that when it is explicitly described that “A and B are connected”,the case where A and B are electrically connected, the case where A andB are functionally connected, and the case where A and B are directlyconnected are included therein. Here, each of A and B is an object(e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer). Accordingly, another elementmay be interposed between elements having a connection relationillustrated in drawings and texts, without limitation to a predeterminedconnection relation, for example, the connection relation illustrated inthe drawings and the texts.

For example, in the case where A and B are electrically connected, oneor more elements which enable electrical connection between A and B(e.g., a switch, a transistor, a capacitor, an inductor, a resistor,and/or a diode) may be connected between A and B. Alternatively, in thecase where A and B are functionally connected, one or more circuitswhich enable functional connection between A and B (e.g., a logiccircuit such as an inverter, a NAND circuit, or a NOR circuit; a signalconverter circuit such as a DA converter circuit, an AD convertercircuit, or a gamma correction circuit; a potential level convertercircuit such as a power supply circuit (e.g., a dc-dc converter, astep-up dc-dc converter, or a step-down dc-dc converter) or a levelshifter circuit for changing a potential level of a signal; a voltagesource; a current source; a switching circuit; an amplifier circuit suchas a circuit which can increase signal amplitude, the amount of current,or the like, an operational amplifier, a differential amplifier circuit,a source follower circuit, or a buffer circuit; a signal generationcircuit; a memory circuit; and/or a control circuit) may be connectedbetween A and B. For example, in the case where a signal output from Ais transmitted to B even when another circuit is interposed between Aand B,A and B are functionally connected.

Note that when it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected with another element or another circuitinterposed therebetween), the case where A and B are functionallyconnected (i.e., the case where A and B are functionally connected withanother circuit interposed therebetween), and the case where A and B aredirectly connected (i.e., the case where A and B are connected withoutanother element or another circuit interposed therebetween) are includedtherein. That is, when it is explicitly described that “A and B areelectrically connected”, the description is the same as the case whereit is explicitly only described that “A and B are connected”.

Note that a display element, a display device which is a deviceincluding a display element, a light-emitting element, and alight-emitting device which is a device including a light-emittingelement can employ various modes. For example, an EL(electroluminescence) element (e.g., an EL element including organic andinorganic materials, an organic EL element, or an inorganic EL element),an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), atransistor (a transistor which emits light depending on the amount ofcurrent), an electron emitter, a liquid crystal element, electronic ink,an electrophoretic element, a grating light valve (GLV), a plasmadisplay panel (PDP), a digital micromirror device (DMD), a piezoelectricceramic display, a carbon nanotube, or the like can be used as a displayelement, a display device, a light-emitting element, or a light-emittingdevice. Such an element can include a display medium whose contrast,luminance, reflectivity, transmittance, or the like changes byelectromagnetic action. Note that display devices having EL elementsinclude an EL display. Display devices having electron emitters includea field emission display (FED), an SED-type flat panel display (SED:surface-conduction electron-emitter display), and the like. Displaydevices having liquid crystal elements include a liquid crystal display(e.g., a transmissive liquid crystal display, a transflective liquidcrystal display, a reflective liquid crystal display, a direct-viewliquid crystal display, or a projection liquid crystal display) and thelike. Display devices having electronic ink or electrophoretic elementsinclude electronic paper.

Note that an EL element is an element including an anode, a cathode, andan EL layer interposed between the anode and the cathode. Note that asan EL layer, a layer utilizing light emission (fluorescence) from asinglet exciton, a layer utilizing light emission (phosphorescence) froma triplet exciton, a layer utilizing light emission (fluorescence) froma singlet exciton and light emission (phosphorescence) from a tripletexciton, a layer formed using an organic material, a layer formed usingan inorganic material, a layer formed using an organic material and aninorganic material, a layer including a high-molecular material, a layerincluding a low-molecular material, a layer including a high-molecularmaterial and a low-molecular material, or the like can be used. Notethat the present invention is not limited to this, and a variety of ELelements can be used as an EL element.

Note that an electron emitter is an element in which electrons areextracted by high electric field concentration on a cathode. Forexample, as an electron emitter, a Spindt type, a carbon nanotube (CNT)type, a metal-insulator-metal (MIM) type in which a metal, an insulator,and a metal are stacked, a metal-insulator-semiconductor (MIS) type inwhich a metal, an insulator, and a semiconductor are stacked, a MOStype, a silicon type, a thin film diode type, a diamond type, a thinfilm type in which a metal, an insulator, a semiconductor, and a metalare stacked, a HEED type, an EL type, a porous silicon type, asurface-conduction (SCE) type, or the like can be used. Note that thepresent invention is not limited to this, and a variety of elements canbe used as an electron emitter.

Note that a liquid crystal element is an element which controlstransmission or non-transmission of light by optical modulation actionof liquid crystals and includes a pair of electrodes and liquidcrystals. Note that the optical modulation action of liquid crystals iscontrolled by an electric filed applied to the liquid crystals(including a horizontal electric field, a vertical electric field, and adiagonal electric field). Note that the following can be used for aliquid crystal element: a nematic liquid crystal, a cholesteric liquidcrystal, a smectic liquid crystal, a discotic liquid crystal, athermotropic liquid crystal, a lyotropic liquid crystal, a low-molecularliquid crystal, a high-molecular liquid crystal, a polymer dispersedliquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, a plasma addressed liquidcrystal (PALC), a banana-shaped liquid crystal, and the like. Inaddition, the following can be used as a diving method of a liquidcrystal: a TN (twisted nematic) mode, an STN (super twisted nematic)mode, an IPS (in-plane-switching) mode, an FFS (fringe field switching)mode, an MVA (multi-domain vertical alignment) mode, a PVA (patternedvertical alignment) mode, an ASV (advanced super view) mode, an ASM(axially symmetric aligned microcell) mode, an OCB (opticallycompensated birefringence) mode, an ECB (electrically controlledbirefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, a guest-host mode, a blue phase mode, and thelike. Note that the present invention is not limited to this, and avariety of liquid crystal elements and driving methods thereof can beused as a liquid crystal element and a driving method thereof.

Note that electronic paper corresponds to a device for displaying imagesby molecules (a device which utilizes optical anisotropy, dye molecularorientation, or the like), a device for displaying images by particles(a device which utilizes electrophoresis, particle movement, particlerotation, phase change, or the like), a device for displaying images bymovement of one end of a film, a device for displaying images by usingcoloring properties or phase change of molecules, a device fordisplaying images by using optical absorption by molecules, or a devicefor displaying images by using self-light emission by combination ofelectrons and holes. For example, the following can be used for adisplay method of electronic paper: microcapsule electrophoresis,horizontal electrophoresis, vertical electrophoresis, a sphericaltwisting ball, a magnetic twisting ball, a columnar twisting ball, acharged toner, an electron powder and granular material, magneticelectrophoresis, a magnetic thermosensitive type, electro wetting,light-scattering (transparent-opaque change), a cholesteric liquidcrystal and a photoconductive layer, a cholesteric liquid crystaldevice, a bistable nematic liquid crystal, a ferroelectric liquidcrystal, a liquid crystal dispersed type with a dichroic dye, a movablefilm, coloring and decoloring properties of a leuco dye, photochromism,electrochromism, electrodeposition, flexible organic EL, and the like.Note that the present invention is not limited to this, and a variety ofelectronic paper and display methods thereof can be used as electronicpaper and a driving method thereof. Here, by using microcapsuleelectrophoresis, defects of electrophoresis, which are aggregation andprecipitation of phoresis particles, can be solved. Electron powder andgranular material has advantages such as high-speed response, highreflectivity, wide viewing angle, low power consumption, and memoryproperties.

Note that a plasma display panel has a structure where a substratehaving a surface provided with an electrode faces with a substratehaving a surface provided with an electrode and a minute groove in whicha phosphor layer is formed at a narrow interval and a rare gas is sealedtherein. Alternatively, the plasma display panel can have a structurewhere a plasma tube is sandwiched between film-form electrodes from thetop and the bottom. The plasma tube is formed by sealing a dischargegas, RGB fluorescent materials, and the like inside a glass tube. Notethat the plasma display panel can perform display by application ofvoltage between the electrodes to generate an ultraviolet ray so that aphosphor emits light. Note that a discharge method may be either a DCmethod or an AC method. Here, as a driving method of the plasma displaypanel, AWS (address while sustain) driving, ADS (address displayseparated) driving in which a subframe is divided into a reset period,an address period, and a sustain period, CLEAR (high-contrast & lowenergy address & reduction of false contour sequence) driving, ALIS(alternate lighting of surfaces) method, TERES (technology of reciprocalsustainer) driving, or the like can be used. Note that the presentinvention is not limited to this, and a variety of driving methods canbe used as a driving method of a plasma display panel.

Note that electroluminescence, a cold cathode fluorescent lamp, a hotcathode fluorescent lamp, an LED, a laser light source, a mercury lamp,or the like can be used as a light source of a display device in which alight source is needed, such as a liquid crystal display (e.g., atransmissive liquid crystal display, a transflective liquid crystaldisplay, a reflective liquid crystal display, a direct-view liquidcrystal display, or a projection liquid crystal display), a displaydevice including a grating light valve (GLV), or a display deviceincluding a digital micromirror device (DMD). Note that the presentinvention is not limited to this, and a variety of light sources can beused as a light source.

Note that a variety of transistors can be used as a transistor, withoutlimitation to a certain type. For example, a thin film transistor (TFT)including a non-single-crystal semiconductor film typified by amorphoussilicon, polycrystalline silicon, microcrystalline (also referred to asmicrocrystal, nanocrystal, or semi-amorphous) silicon, or the like canbe used. In the case of using the TFT, there are various advantages. Forexample, since the TFT can be formed at temperature lower than that ofthe case of using single crystal silicon, manufacturing cost can bereduced or a manufacturing apparatus can be made larger. Since themanufacturing apparatus can be made larger, the TFT can be formed usinga large substrate. Therefore, many display devices can be formed at thesame time at low cost. In addition, since the manufacturing temperatureis low, a substrate having low heat resistance can be used. Therefore,the transistor can be formed using a light-transmitting substrate.Further, transmission of light in a display element can be controlled byusing the transistor formed using the light-transmitting substrate.Alternatively, part of a film included in the transistor can transmitlight because the thickness of the transistor is small. Therefore, theaperture ratio can be improved.

Note that by using a catalyst (e.g., nickel) in the case of formingpolycrystalline silicon, crystallinity can be further improved and atransistor having excellent electrical characteristics can be formed.Accordingly, a gate driver circuit (e.g., a scan line driver circuit), asource driver circuit (e.g., a signal line driver circuit), and/or asignal processing circuit (e.g., a signal generation circuit, a gammacorrection circuit, or a DA converter circuit) can be formed using thesame substrate as a pixel portion.

Note that by using a catalyst (e.g., nickel) in the case of formingmicrocrystalline silicon, crystallinity can be further improved and atransistor having excellent electrical characteristics can be formed. Inthis case, crystallinity can be improved by just performing heattreatment without performing laser irradiation. Accordingly, a gatedriver circuit (e.g., a scan line driver circuit) and part of a sourcedriver circuit (e.g., an analog switch) can be formed using the samesubstrate as a pixel portion. In addition, in the case of not performinglaser irradiation for crystallization, unevenness in crystallinity ofsilicon can be suppressed. Therefore, high-quality images can bedisplayed.

Note that polycrystalline silicon and microcrystalline silicon can beformed without using a catalyst (e.g., nickel).

Note that it is preferable that crystallinity of silicon be improved topolycrystal, microcrystal, or the like in the whole panel; however, thepresent invention is not limited to this. Crystallinity of silicon maybe improved only in part of the panel. Selective improvement incrystallinity is possible by selective laser irradiation or the like.For example, only a peripheral driver circuit region excluding pixelsmay be irradiated with laser light. Alternatively, only a region of agate driver circuit, a source driver circuit, or the like may beirradiated with laser light. Alternatively, only part of a source drivercircuit (e.g., an analog switch) may be irradiated with laser light.Accordingly, crystallinity of silicon can be improved only in a regionin which a circuit needs to be operated at high speed. Since a pixelregion is not particularly needed to be operated at high speed, even ifcrystallinity is not improved, the pixel circuit can be operated withoutproblems. Since a region whose crystallinity is improved is small,manufacturing steps can be decreased, throughput can be increased, andmanufacturing cost can be reduced. Since the number of necessarymanufacturing apparatus is small, manufacturing cost can be reduced.

A transistor can be formed using a semiconductor substrate, an SOIsubstrate, or the like. Thus, a transistor with few variations incharacteristics, sizes, shapes, or the like, with high current supplycapability, and with a small size can be formed. By using such atransistor, power consumption of a circuit can be reduced or a circuitcan be highly integrated.

A transistor including a compound semiconductor or an oxidesemiconductor, such as zinc oxide (ZnO), an oxide containing indium,gallium, and zinc (InGaZnO), silicon germanium (SiGe), gallium arsenide(GaAs), indium zinc oxide (IZO), indium tin oxide (ITO), or tin oxide(SnO), a thin film transistor obtained by thinning such a compoundsemiconductor or an oxide semiconductor, or the like cab be used. Thus,manufacturing temperature can be lowered and for example, such atransistor can be formed at room temperature. Accordingly, thetransistor can be formed directly on a substrate having low heatresistance, such as a plastic substrate or a film substrate. Note thatsuch a compound semiconductor or an oxide semiconductor can be used notonly for a channel portion of the transistor but also for otherapplications. For example, such a compound semiconductor or an oxidesemiconductor can be used for a resistor, a pixel electrode, or alight-transmitting electrode. Further, since such an element can beformed at the same time as the transistor, cost can be reduced.

A transistor or the like formed by an inkjet method or a printing methodcan be used. Thus, a transistor can be formed at room temperature, canbe formed at a low vacuum, or can be formed using a large substrate.Since the transistor can be formed without using a mask (reticle), thelayout of the transistor can be easily changed. Further, since it is notnecessary to use a resist, material cost is reduced and the number ofsteps can be reduced. Furthermore, since a film is formed only in anecessary portion, a material is not wasted as compared to amanufacturing method by which etching is performed after the film isformed over the entire surface, so that cost can be reduced.

A transistor or the like including an organic semiconductor or a carbonnanotube can be used. Thus, such a transistor can be formed over aflexible substrate. A semiconductor device formed using such a substratecan resist shocks.

Further, transistors with a variety of structures can be used. Forexample, a MOS transistor, a junction transistor, a bipolar transistor,or the like can be used as a transistor. By using a MOS transistor, thesize of the transistor can be reduced. Thus, a plurality of transistorscan be mounted. By using a bipolar transistor, large current can flowThus, a circuit can be operated at high speed.

Note that a MOS transistor, a bipolar transistor, and the like may beformed over one substrate. Thus, reduction in power consumption,reduction in size, high-speed operation, and the like can be achieved.

Note that the structure of a transistor can be a variety of structures,without limitation to a certain structure. For example, a multi-gatestructure having two or more gate electrodes can be used. By using themulti-gate structure, a structure where a plurality of transistors areconnected in series is provided because channel regions are connected inseries. With the multi-gate structure, the amount of off-state currentcan be reduced and the withstand voltage of the transistor can beincreased (reliability can be improved). Further, with the multi-gatestructure, drain-source current does not fluctuate very much even whendrain-source voltage fluctuates when the transistor operates in asaturation region, so that a flat slope of voltage-currentcharacteristics can be obtained. By utilizing the flat slope of thevoltage-current characteristics, an ideal current source circuit or anactive load having an extremely large resistance value can be realized.Accordingly, a differential circuit or a current mirror circuit havingexcellent properties can be realized.

As another example, a structure where gate electrodes are formed aboveand below a channel can be used. By using the structure where gateelectrodes are formed above and below the channel, a channel region isincreased, so that the amount of current can be increased.Alternatively, by using the structure where gate electrodes are formedabove and below the channel, a depletion layer can be easily formed, sothat subthreshold swing can be improved. Note that when the gateelectrodes are formed above and below the channel, a structure where aplurality of transistors are connected in parallel is provided.

A structure where a gate electrode is formed above a channel region, astructure where a gate electrode is formed below a channel region, astaggered structure, an inverted staggered structure, a structure wherea channel region is divided into a plurality of regions, or a structurewhere channel regions are connected in parallel or in series can beused. Alternatively, a structure where a source electrode or a drainelectrode overlaps with a channel region (or part of it) can be used. Byusing the structure where the source electrode or the drain electrodeoverlaps with the channel region (or part of it), unstable operation dueto accumulation of electric charge in part of the channel region can beprevented. Alternatively, a structure where an LDD region is providedcan be used. By providing the LDD region, the amount of off statecurrent can be reduced or the withstand voltage of the transistor can beincreased (reliability can be improved). Further, by providing the LDDregion, drain-source current does not fluctuate very much even whendrain-source voltage fluctuates when the transistor operates in thesaturation region, so that a flat slope of voltage-currentcharacteristics can be obtained.

Note that a variety of transistors can be used as a transistor, and thetransistor can be formed using a variety of substrates. Accordingly, allthe circuits that are necessary to realize a predetermined function canbe formed using the same substrate. For example, all the circuits thatare necessary to realize the predetermined function can be formed usinga glass substrate, a plastic substrate, a single crystal substrate, anSOI substrate, or any other substrate. When all the circuits that arenecessary to realize the predetermined function are formed using thesame substrate, cost can be reduced by reduction in the number ofcomponents or reliability can be improved by reduction in the number ofconnections to circuit components. Alternatively, some of the circuitswhich are necessary to realize the predetermined function can be formedusing one substrate and some of the circuits which are necessary torealize the predetermined function can be formed using anothersubstrate. That is, not all the circuits that are necessary to realizethe predetermined function are required to be formed using the samesubstrate. For example, some of the circuits which are necessary torealize the predetermined function can be formed by transistors using aglass substrate and some of the circuits which are necessary to realizethe predetermined function can be formed using a single crystalsubstrate, so that an IC chip formed by a transistor using the singlecrystal substrate can be connected to the glass substrate by COG (chipon glass) and the IC chip may be provided over the glass substrate.Alternatively, the IC chip can be connected to the glass substrate byTAB (tape automated bonding) or a printed wiring board. When some of thecircuits are formed using the same substrate in this manner, cost can bereduced by reduction in the number of components or reliability can beimproved by reduction in the number of connections to circuitcomponents. Alternatively, when circuits with high driving voltage andhigh driving frequency, which consume large power, are formed using asingle crystal substrate instead of forming such circuits using the samesubstrate, and an IC chip formed by the circuits is used, for example,increase in power consumption can be prevented.

Note that one pixel corresponds to one element whose brightness can becontrolled. Therefore, for example, one pixel corresponds to one colorelement and brightness is expressed with the one color element.Accordingly, in that case, in the case of a color display device havingcolor elements of R (red), G (green), and B (blue), the minimum unit ofan image is formed of three pixels of an R pixel, a G pixel, and a Bpixel. Note that the color elements are not limited to three colors, andcolor elements of more than three colors may be used or a color otherthan RGB may be used. For example, RGBW (W corresponds to white) can beused by adding white. Alternatively, one or more colors of yellow, cyan,magenta, emerald green, vermilion, and the like can be added to RGB.Alternatively, a color similar to at least one of R, G and B can beadded to RGB. For example, R, G, B1, and B2 may be used. Although bothB1 and B2 are blue, they have different frequencies. In a similarmanner, R1, R2, G, and B can be used. By using such color elements,display which is closer to the real object can be performed and powerconsumption can be reduced. As another example, in the case ofcontrolling brightness of one color element by using a plurality ofregions, one region can correspond to one pixel. Therefore, for example,in the case of performing area ratio gray scale display or in the caseof including a subpixel, a plurality of regions which control brightnessare provided in each color element and gray levels are expressed withthe whole regions. In this case, one region which controls brightnesscan correspond to one pixel. Thus, in that case, one color elementincludes a plurality of pixels. Alternatively, even when the pluralityof regions which control brightness are provided in one color element,these regions may be collected and one color element may be referred toas one pixel. Thus, in that case, one color element includes one pixel.Alternatively, in the case where brightness is controlled in a pluralityof regions in each color element, the size of regions which contributeto display is varied depending on pixels in some cases. Alternatively,in the plurality of regions which control brightness in each colorelement, signals supplied to each of the plurality of regions may beslightly varied so that the viewing angle is widened. That is,potentials of pixel electrodes included in the plurality of regionsprovided in each color element can be different from each other.Accordingly, voltage applied to liquid crystal molecules are varieddepending on the pixel electrodes. Therefore, the viewing angle can bewidened.

Note that explicit description “one pixel (for three colors)”corresponds to the case where three pixels of R, G, and B are consideredas one pixel. Explicit description “one pixel (for one color)”corresponds to the case where the plurality of regions are provided ineach color element and collectively considered as one pixel.

Note that pixels are provided (arranged) in matrix in some cases. Here,description that pixels are provided (arranged) in matrix includes thecase where the pixels are arranged in a straight line and the case wherethe pixels are arranged in a jagged line, in a longitudinal direction ora lateral direction. Thus, for example, in the case of performing fullcolor display with three color elements (e.g., RGB), the following casesare included: the case where the pixels are arranged in stripes and thecase where dots of the three color elements are arranged in a deltapattern. In addition, the case is also included in which dots of thethree color elements are provided in Bayer arrangement. Note that thesize of display regions may be different between dots of color elements.Thus, power consumption can be reduced or the life of a display elementcan be prolonged.

Note that an active matrix method in which an active element is includedin a pixel or a passive matrix method in which an active element is notincluded in a pixel can be used.

In an active matrix method, as an active element (a non-linear element),not only a transistor but also a variety of active elements (non-linearelements) can be used. For example, an MIM (metal insulator metal), aTFD (thin film diode), or the like can also be used. Since such anelement has few number of manufacturing steps, manufacturing cost can bereduced or yield can be improved. Further, since the size of the elementis small, the aperture ratio can be improved, so that power consumptioncan be reduced or higher luminance can be achieved.

Note that as a method other than the active matrix method, a passivematrix method in which an active element (a non-linear element) is notused can be used. Since an active element (a non-linear element) is notused, manufacturing steps is few, so that manufacturing cost can bereduced or yield can be improved. Further, since an active element (anon-linear element) is not used, the aperture ratio can be improved, sothat power consumption can be reduced or higher luminance can beachieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor has a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor change depending on thestructure, the operating condition, and the like of the transistor, itis difficult to define which is a source or a drain. Thus, a regionwhich serves as a source and a drain is not referred to as a source or adrain in some cases. In such a case, one of the source and the drainmight be referred to as a first terminal and the other of the source andthe drain might be referred to as a second terminal, for example.Alternatively, one of the source and the drain might be referred to as afirst electrode and the other of the source and the drain might bereferred to as a second electrode. Alternatively, one of the source andthe drain might be referred to as a first region and the other of thesource and the drain might be referred to as a second region.

Note that a transistor may be an element having at least three terminalsof a base, an emitter, and a collector. In this case, in a similarmanner, one of the emitter and the collector might be referred to as afirst terminal and the other of the emitter and the collector might bereferred to as a second terminal.

Note that a gate corresponds to all or some of a gate electrode and agate wiring (also referred to as a gate line, a gate signal line, a scanline, a scan signal line, or the like). A gate electrode corresponds topart of a conductive film which overlaps with a semiconductor whichforms a channel region with a gate insulating film interposedtherebetween. Note that part of the gate electrode overlaps with an LDD(lightly doped drain) region or a source region (or a drain region) withthe gate insulating film interposed therebetween in some cases. A gatewiring corresponds to a wiring for connecting gate electrodes oftransistors to each other, a wiring for connecting gate electrodes ofpixels to each other, or a wiring for connecting a gate electrode toanother wiring.

Note that a source corresponds to all or some of a source region, asource electrode, and a source wiring (also referred to as a sourceline, a source signal line, a data line, a data signal line, or thelike). A source region corresponds to a semiconductor region containinga large amount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Therefore, a region containinga small amount of p-type impurities or n-type impurities, namely, an LDD(lightly doped drain) region is not included in the source region. Asource electrode is part of a conductive layer which is formed using amaterial different from that of a source region and is electricallyconnected to the source region. However, a source electrode and a sourceregion are collectively referred to as a source electrode in some cases.A source wiring corresponds to a wiring for connecting source electrodesof transistors to each other, a wiring for connecting source electrodesof pixels to each other, or a wiring for connecting a source electrodeto another wiring.

Note that the same can be said for a drain.

Note that a semiconductor device corresponds to a device having acircuit including a semiconductor element (e.g., a transistor, a diode,or a thyristor). The semiconductor device may also correspond to alldevices that can function by utilizing semiconductor characteristics. Inaddition, the semiconductor device corresponds to a device having asemiconductor material.

Note that a display device corresponds to a device having a displayelement. The display device may include a plurality of pixels eachhaving a display element. Note that that the display device may includea peripheral driver circuit for driving the plurality of pixels. Theperipheral driver circuit for driving the plurality of pixels may beformed using the same substrate as the plurality of pixels. The displaydevice may include a peripheral driver circuit provided over a substrateby wire bonding or bump bonding, namely, an IC chip connected by chip onglass (COG) or an IC chip connected by TAB or the like. The displaydevice may include a flexible printed circuit (FPC) to which an IC chip,a resistor, a capacitor, an inductor, a transistor, or the like isattached. Note that the display device may include a printed wiringboard (PWB) which is connected through a flexible printed circuit (FPC)and to which an IC chip, a resistor, a capacitor, an inductor, atransistor, or the like is attached. The display device may include anoptical sheet such as a polarizing plate or a retardation plate. Thedisplay device may include a lighting device, a housing, an audio inputand output device, an optical sensor, or the like.

Note that a lighting device may include a backlight unit, a light guideplate, a prism sheet, a diffusion sheet, a reflective sheet, a lightsource (e.g., an LED or a cold cathode fluorescent lamp), a coolingdevice (e.g., a water cooling device or an air cooling device), or thelike.

Note that a light-emitting device corresponds to a device having alight-emitting element or the like. In the case where a light-emittingdevice includes a light-emitting element as a display element, thelight-emitting device is one of specific examples of a display device.

Note that a reflective device corresponds to a device having alight-reflective element, a light diffraction element, light-reflectiveelectrode, or the like.

Note that a liquid crystal display device corresponds to a displaydevice including a liquid crystal element. Liquid crystal displaydevices include a direct-view liquid crystal display, a projectionliquid crystal display, a transmissive liquid crystal display, areflective liquid crystal display, a transflective liquid crystaldisplay, and the like.

Note that a driving device corresponds to a device having asemiconductor element, an electric circuit, or an electronic circuit.For example, a transistor which controls input of signals from a sourcesignal line to pixels (also referred to as a selection transistor, aswitching transistor, or the like), a transistor which supplies voltageor current to a pixel electrode, a transistor which supplies voltage orcurrent to a light-emitting element, and the like are examples of thedriving device. A circuit which supplies signals to a gate signal line(also referred to as a gate driver, a gate line driver circuit, or thelike), a circuit which supplies signals to a source signal line (alsoreferred to as a source driver, a source line driver circuit, or thelike), and the like are also examples of the driving device.

Note that a display device, a semiconductor device, a lighting device, acooling device, a light-emitting device, a reflective device, a drivingdevice, and the like overlap with each other in some cases. For example,a display device includes a semiconductor device and a light-emittingdevice in some cases. Alternatively, a semiconductor device includes adisplay device and a driving device in some cases.

According to one embodiment of the invention disclosed in thisspecification, deterioration in a transistor can be suppressed.

Alternatively, according to one embodiment of the invention disclosed inthis specification, a malfunction of a circuit due to deterioration in atransistor can be suppressed.

Alternatively, according to one embodiment of the invention disclosed inthis specification, reliability of a circuit including a transistor canbe improved.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C illustrate examples of a semiconductor device;

FIG. 2A illustrates a semiconductor device, and FIGS. 2B to 2Fillustrate a driving method of the semiconductor device;

FIGS. 3A to 3E illustrate the semiconductor device and a driving methodthereof;

FIG. 4A illustrates a semiconductor device, and FIGS. 4B to 4Fillustrate a driving method of the semiconductor device;

FIGS. 5A to 5C illustrate examples of a semiconductor device;

FIGS. 6A to 6G illustrate an example of operation of the semiconductordevice;

FIGS. 7A to 7G illustrate an example of operation of the semiconductordevice;

FIGS. 8A to 8G illustrate an example of operation of the semiconductordevice;

FIGS. 9A to 9G illustrate an example of operation of the semiconductordevice;

FIGS. 10A to 10G illustrate an example of operation of the semiconductordevice;

FIGS. 11A to 11G illustrate an example of operation of the semiconductordevice;

FIGS. 12A to 12C illustrate examples of a semiconductor device;

FIGS. 13A to 13H illustrate an example of operation of the semiconductordevice;

FIGS. 14A to 14H illustrate an example of operation of the semiconductordevice;

FIGS. 15A to 15H illustrate an example of operation of the semiconductordevice;

FIG. 16 illustrates an example of a semiconductor device:

FIG. 17 illustrates an example of a semiconductor device;

FIG. 18 illustrates an example of a semiconductor device;

FIG. 19 illustrates an example of a semiconductor device;

FIG. 20 illustrates an example of a semiconductor device;

FIGS. 21A to 21C illustrate examples of a semiconductor device;

FIGS. 22A to 22C illustrate examples of a semiconductor device;

FIG. 23 illustrates an example of a semiconductor device;

FIG. 24 illustrates an example of a semiconductor device;

FIG. 25 illustrates an example of a semiconductor device;

FIG. 26 illustrates an example of a semiconductor device;

FIGS. 27A to 27D illustrate an example of a semiconductor device; and

FIGS. 28A to 28E illustrate applications of a semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to thedrawings. Note that the embodiments can be implemented in variousdifferent ways and it will be readily appreciated by those skilled inthe art that modes and details of the embodiments can be changed invarious ways without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the following description of the embodiments. Note thatin structures described below, the same portions or portions havingsimilar functions are denoted by common reference numerals in differentdrawings, and description thereof is not repeated.

Further, a content (or may be part of the content) described in oneembodiment may be applied to, combined with, or replaced by a differentcontent (or may be part of the different content) described in theembodiment and/or a content (or may be part of the content) described inone or a plurality of different embodiments.

Note that in each embodiment, a content described in the embodiment is acontent described with reference to a variety of diagrams or a contentdescribed with a text described in this specification.

Note that by combining a diagram (or may be part of the diagram)illustrated in one embodiment with another pan of the diagram, adifferent diagram (or may be part of the different diagram) illustratedin the embodiment, and/or a diagram (or may be part of the diagram)illustrated in one or a plurality of different embodiments, much morediagrams can be formed.

Note that in a diagram or a text described in one embodiment, part ofthe diagram or the text is taken out, and one embodiment of theinvention can be constituted. Thus, in the case where a diagram or atext related to a certain portion is described, the context taken outfrom part of the diagram or the text is also disclosed as one embodimentof the invention, and one embodiment of the invention can beconstituted. Therefore, for example, in a diagram (e.g., across-sectional view, a plan view, a circuit diagram, a block diagram, aflow chart, a process diagram, a perspective view, a cubic diagram, alayout diagram, a timing chart, a structure diagram, a schematic view, agraph, a list, a ray diagram, a vector diagram, a phase diagram, awaveform chart, a photograph, or a chemical formula) or a text in whichone or more active elements (e.g., transistors or diodes), wirings,passive elements (e.g., capacitors or resistors), conductive layers,insulating layers, semiconductor layers, organic materials, inorganicmaterials, components, substrates, modules, devices, solids, liquids,gases, operating methods, manufacturing methods, or the like aredescribed, part of the diagram or the text is taken out, and oneembodiment of the invention can be constituted. For example, M pieces ofcircuit elements (e.g., transistors or capacitors) (M is an integer,where M<N) are taken out from a circuit diagram in which N pieces ofcircuit elements (e.g., transistors or capacitors) (N is an integer) areprovided, and one embodiment of the invention can be constituted. Asanother example, M pieces of layers (M is an integer, where M<N) aretaken out from a cross-sectional view in which N pieces of layers (N isan integer) are provided, and one embodiment of the invention can beconstituted. As another example, M pieces of elements (M is an integer,where M<N) are taken out from a flow chart in which N pieces of elements(N is an integer) are provided, and one embodiment of the invention canbe constituted.

Note that in a diagram or a text described in one embodiment, in thecase where at least one specific example is described, it will bereadily appreciated by those skilled in the art that a broader conceptof the specific example can be derived. Therefore, in the diagram or thetext described in one embodiment, in the case where at least onespecific example is described, a broader concept of the specific exampleis disclosed as one embodiment of the invention, and one embodiment ofthe invention can be constituted.

Note that a content described in at least a diagram (or may be part ofthe diagram) is disclosed as one embodiment of the invention, and oneembodiment of the invention can be constituted. Therefore, when acertain content is described in a diagram, the content is disclosed asone embodiment of the invention even when the content is not describedwith a text, and one embodiment of the invention can be constituted. Ina similar manner, part of a diagram, which is taken out from thediagram, is disclosed as one embodiment of the invention, and oneembodiment of the invention can be constituted.

Note that it might be possible for those skilled in the art toconstitute one embodiment of the invention even when portions to whichall terminals of an active element (e.g., a transistor or a diode), apassive element (e.g., a capacitor or a resistor), or the like areconnected are not specified. In particular, in the case where the numberof portions to which the terminal is connected is plural, it is notnecessary to specify the portions to which the terminal is connected.Therefore, it might be possible to constitute one embodiment of theinvention by specifying only portions to which some of terminals of anactive element (e.g., a transistor or a diode), a passive element (e.g.,a capacitor or a resistor), or the like are connected.

Note that it might be possible for those skilled in the art to specifythe invention when at least a connection portion of a circuit isspecified. Alternatively, it might be possible for those skilled in theart to specify the invention when at least a function of a circuit isspecified. Therefore, when a connection portion of a circuit isspecified, the circuit is disclosed as one embodiment of the inventioneven when a function is not specified, and one embodiment of theinvention can be constituted. Alternatively, when a function of acircuit is specified, the circuit is disclosed as one embodiment of theinvention even when a connection portion is not specified, and oneembodiment of the invention can be constituted.

Embodiment 1

In this embodiment, examples of a semiconductor device including atransistor are described.

In order to suppress deterioration in a transistor, in the semiconductordevice described in this embodiment, the direction of current flowingthrough the transistor is changed (inverted) in a period during whichthe transistor is on. That is, by changing the level of voltage appliedto a first terminal and a second terminal (terminals serving as a sourceand a drain) of the transistor in the period during which the transistoris on every given period, the source and the drain are switched everygiven period. Specific circuit structures and operation are describedbelow with reference to drawings.

The semiconductor device described in this embodiment includes at leasta transistor 111 provided between a wiring 101 and a wiring 103 and atransistor 112 provided between a wiring 102 and the wiring 103 (seeFIG. 1A).

One of a source and a drain of the transistor 111 is electricallyconnected to the wiring 101, and the other of the source and the drainof the transistor 111 is electrically connected to the wiring 103. Byturning on the transistor 111, a signal which is input to the wiring 101(IN1) is supplied to the wiring 103. One of a source and a drain of thetransistor 112 is electrically connected to the wiring 102, and theother of the source and the drain of the transistor 112 is electricallyconnected to the wiring 103. By turning on the transistor 112, a signalwhich is input to the wiring 102 (IN2) is supplied to the wiring 103.

That is, a first signal corresponding to the signal which is input tothe wiring 101 (IN1) or a second signal corresponding to the signalwhich is input to the wiring 102 (IN2) is supplied to the wiring 103.

For example, by using a high potential (a high (H)-level signal) and alow potential (a low (L)-level signal) as the first signal and thesecond signal and by controlling on/off of the transistor 11 and thetransistor 112, the H-level signal or the L-level signal can beselectively output to the wiring 103. Alternatively, as illustrated inFIG. 2A, by connecting a gate of a transistor to the wiring 103 andoutputting the H-level signal or the L-level signal from the wiring 103,on/off of the transistor can be controlled.

Note that although n-channel transistors are used as the transistor 111and the transistor 112 in FIGS. 1A to 1C, p-channel transistors may beused. Alternatively, the polarities of the transistor 111 and thetransistor 112 may be different from each other, or a CMOS may be usedfor each transistor. Further, the transistor 111 can serve as a switchprovided between the wiring 101 and the wiring 103, and the transistor112 can serve as a switch provided between the wiring 102 and the wiring103 (see FIG. 1B).

In this embodiment, in the structures illustrated in FIGS. 1A to 1C, inat least one of the transistor 111 and the transistor 112, the directionof current flowing through the transistor is changed. That is, the levelof voltage applied to a first terminal and a second terminal (terminalsserving as the source and the drain) of at least one of the transistor111 and the transistor 112 is changed every given period (the source andthe drain are switched).

In particular, in operating a circuit, in a transistor which is kept onfor a long time, it is preferable that the direction of current flowingthrough the transistor be changed. For example, in the case where thetransistor 112 is kept on for a long time in FIG. 1A, at least thedirection of current flowing through the transistor 112 (a direction Aor a direction B) is changed. That is, the level of voltage applied tothe first terminal and the second terminal of the transistor 112 ischanged every period (the source and the drain are switched).

A specific operating method is described below with reference to FIGS.2A to 2F and FIGS. 3A to 3E.

In the following description, a structure where a gate of an n-channeltransistor 121 is electrically connected to the wiring 103 (e.g., astructure where the wiring 103 serves as a gate line) is described (seeFIG. 2A). In addition, the case is described in which the direction ofcurrent flowing through the transistor 112 is changed in a period duringwhich the transistor 112 is on by using a signal whose potential ischanged every predetermined period as a signal which is input to thewiring 102.

FIGS. 2B to 2F illustrate a signal which is supplied to the wiring 103(Out), the signal which is input to the wiring 101 (IN1), the signalwhich is input to the wiring 102 (IN2), a signal which is input to agate of the transistor 111 (IN3), and a signal which is input to a gateof the transistor 112 (IN4), respectively. Needless to say, thesesignals (IN1 to IN4) are just examples, and signals are not limited tothe signals illustrated in FIGS. 2A to 2F.

First, in a period T1, the signal for turning on the transistor 111(IN3) is input to the gate of the transistor 111. Accordingly, thetransistor 111 is turned on, and the first signal corresponding to thesignal which is input to the wiring 101 (IN1) (here, an H-level signal(a selection signal for turning on the transistor 121)) is supplied tothe wiring 103 through the transistor 111. Then, the selection signal isinput to the gate of the transistor 121 which is connected to the wiring103, so that the transistor 121 is turned on (see FIG. 3A).

In the period T1, in the case where a potential of the signal which isinput to the gate of the transistor 111 (IN3) is V_(H), when it isassumed that the threshold voltage of the transistor 111 is Vth, apotential of a signal which is output to the wiring 103 is V_(H)−Vth. Inorder to set the potential of the signal which is output to the wiring103 at V_(H), the potential of the signal which is input to the gate ofthe transistor 111 (IN3) is set higher than V_(H)+Vth by setting thegate of the transistor 111 in a floating state in the period T1 andperforming bootstrap operation. Needless to say, in order to set thepotential of the signal which is output to the wiring 103 at V_(H), thepotential of the signal which is input to the gate of the transistor 111(IN3) may be set higher than V_(H)+Vth (e.g., V_(H)+Vth+α) in advance.

In addition, in the period T1, the transistor 112 is brought out ofconduction (off). Note that this embodiment is not limited to this, andthe transistor 112 may be on as long as the selection signal is outputto the wiring 103. In this case, a potential of the signal which isinput to the wiring 102 (IN2) is preferably V_(H).

Alternatively, in a period before the period T1, the transistor 111 maybe on. In this case, the signal which is input to the wiring 101 ispreferably an L-level signal.

Next, in a period T2, the signal for turning on the transistor 112 (IN4)is input to the gate of the transistor 112. In this case, in thetransistor 112, a potential of a terminal which is connected to thewiring 102 (in this case, V_(LL)) is lower than a potential of aterminal which is connected to the wiring 103 (in this case, V_(H)), sothat the terminal which is connected to the wiring 102 is the source andthe terminal which is connected to the wiring 103 is the drain.Accordingly, a gate-source potential of the transistor 112(VgsB=V_(H)−V_(LL)) is higher than the threshold voltage of thetransistor 112, so that the transistor 112 is turned on. Thus, thesecond signal corresponding to the signal which is input to the wiring102 (IN2) (here, a non-selection signal for turning off the transistor121 (having the potential V_(LL))) is supplied to the wiring 103 throughthe transistor 112.

Then, the non-selection signal is supplied to the gate of the transistor121 which is connected to the wiring 103, so that the transistor 121 isturned off. Note that in the period T2, the transistor 111 is off. Notethat the state of the transistor 111 is not limited to this, and thetransistor 111 may be on as long as the signal IN1 has the potentialV_(LL).

In this manner, in the period T2, in the transistor 112, the potentialof the terminal which is connected to the wiring 102 is lower than thepotential of the terminal which is connected to the wiring 103, so thatthe terminal which is connected to the wiring 102 is the source and theterminal which is connected to the wiring 103 is the drain. Currentflows from the drain to the source (in a direction B in FIG. 2A) (seeFIG. 3B).

Next, in a period T3, the transistor 111 is kept off, and the potentialof the signal which is input to the wiring 102 (IN2) is changed fromV_(LL) to V_(LH) (V_(LL)<V_(LH)). In this case, in the transistor 112,the potential of the terminal which is connected to the wiring 102 (inthis case, V_(LH)) is higher than the potential of the terminal which isconnected to the wiring 103 (in this case, V_(LL)), so that the terminalwhich is connected to the wiring 102 is the drain and the terminal whichis connected to the wiring 103 is the source. Since the gate-sourcepotential of the transistor 112 (VgsA=V_(H)−V_(LH)) is kept higher thanthe threshold voltage of the transistor 112, the transistor 112 is kepton, and the second signal corresponding to the signal which is input tothe wiring 102 (IN2) (here, the non-selection signal for turning off thetransistor 121 (having the potential V_(LH))) is supplied to the wiring103 through the transistor 112.

Then, since the non-selection signal is input to the gate of thetransistor 121 which is connected to the wiring 103, the transistor 121is kept off. Here, the potential Vi and the potential V_(LL) aredifferent from each other and are potentials which do not turn on thetransistor 121 even when they are applied to the gate of the transistor121. For example, when the lowest potential in a source or a drain ofthe transistor 121 is Vmin, the highest voltage is Vmax, and thethreshold voltage of the transistor 121 is Vth, V_(LH)−Vmin<Vth andV_(H)−Vmax>Vth are satisfied.

In this manner, in the period T13, in the transistor 112, the potentialof the terminal which is connected to the wiring 102 is higher than thepotential of the terminal which is connected to the wiring 103, so thatthe terminal which is connected to the wiring 102 is the drain and theterminal which is connected to the wiring 103 is the source. Currentflows from the drain to the source (in a direction A in FIG. 2A) (seeFIG. 3C).

In consecutive periods Tn and T (n+1), the operation in the period T2and the period T3 is repeated. Thus, a second signal whose potential isswitched between the potential V_(LL) and the potential V_(LH) everypredetermined period is supplied to the wiring 103 (here, a signal whosepotential is V_(LH) and is V_(LL) in even-numbered periods is supplied),and the transistor 121 is kept off. Further, although the transistor 112is kept on, the direction of current is changed (see FIGS. 3D and 3E).

In other words, in this embodiment, the level of voltage of the twoterminals serving as the source and the drain of the transistor 112 ischanged by input of an L-level signal (a non-selection signal) whosepotential is changed every given period to the transistor 112 whichcontinuously outputs L-level signals, so that the direction of currentflowing through the transistor 112 is changed.

With a structure in which the direction of current flowing through thetransistor 112 is changed (the source and the drain are switched) inthis manner, even in the case where the transistor 112 is on for a longtime in order to stably turn off the transistor 121, concentration of anelectric field on a channel portion (an end of the drain) of thetransistor 112 is relieved. Thus, deterioration in the transistor 112can be suppressed. Accordingly, a malfunction of the circuit due todeterioration in the transistor is suppressed, so that reliability canbe improved.

In particular, in the case where amorphous silicon or microcrystallinesilicon (microcrystal silicon or nanocrystal silicon) is used for achannel formation region of a transistor, it is effective to operate atransistor which is kept on for a long time in an operating period of acircuit as illustrated in FIGS. 2A to 2F or FIGS. 3A to 3E insuppressing deterioration in the transistor. Note that besides amorphoussilicon or microcrystalline silicon, it is effective to use, forexample, polysilicon, an oxide semiconductor (e.g., ZnO or IGZO(InGaZnO)), an organic semiconductor, a carbon nanotube, or the like fora channel formation region in suppressing deterioration in thetransistor.

Note that although FIGS. 2A to 2F illustrate the case where a signalwhose potential is switched between a first potential V_(LH) and asecond potential V_(LL) (V_(LH)>V_(LL)) every given period is used as anL-level signal, signals which are input to the wiring 102 are notlimited to two kinds of signals having different potentials. Three ormore kinds of different potentials may be input to the transistor 112 incombination as long as the potentials are not potentials for turning onthe transistor 121.

In addition, although FIGS. 2A to 2F illustrate the case where the firstpotential V_(LH) and the second potential V_(LL) are alternately andrepeatedly input to the wiring 102 for the same length of time, periodsfor inputting the first potential and the second potential can be setoptionally. Further, even in the case where the first potential V_(LH)and the second potential V_(LL) are alternately and repeatedly input tothe wiring 102 for the same length of time, the cycle can be setoptionally.

In addition, the signals illustrated in FIGS. 2A to 2F (IN1 to IN4) arejust examples, and this embodiment is not limited to this. For example,although FIGS. 2A to 2F illustrate the case where a signal having aconstant potential is input to the wiring 101 (e.g., the case where thewiring 101 is connected to a power supply line), a different signal(e.g., a clock signal) may be input to the wiring 101. Further, althoughFIGS. 2A to 2F illustrate the case where the transistor 111 is off inthe period T2 to the period Tn, a period during which the transistor 111is on may be provided in the case where an L-level signal is suppliedfrom the wiring 101.

In addition, although the case where the potential of the signal whichis input to the gate of the transistor 112 (IN4) is V_(LL) in the periodT1 is described, the potential of the signal is not limited to this aslong as the transistor 112 is turned off. For example, a potential whichas lower than V_(LL) may be used as the potential of the signal which isinput to the gate of the transistor 112 (IN4). In this case, since Vgscan be made lower than 0 V when the transistor 112 is off, deteriorationin the transistor 112 can be effectively suppressed.

In addition, although FIGS. 2A to 2F illustrate the case where thetransistor 112 is kept on in the period T12 to the period Tn, the stateof the transistor 112 is not limited to this. For example, thetransistor 112 may be turned off in some of the period T2 to the periodTn. That is, a period during which the transistor 112 is on is combinedwith a period during which the transistor 112 is off, and the directionof current flowing through the transistor 112 is changed in the periodduring which the transistor 112 is on. In this case, deterioration inthe transistor 112 can be more effectively suppressed Note that thepotentials of the wiring 102 and the wiring 103 may be the same ordifferent in the period during which the transistor 112 is off. Further,although the period during which the transistor 112 is off is notparticularly limited to a certain period, it is preferable that theperiod during which the transistor 112 is on and the period during whichthe transistor 112 is off be the same or substantially the same in viewof controllability.

In addition, the semiconductor device described in this embodiment canhave a circuit structure in which bootstrap operation is performedutilizing capacitive coupling between the gate and the source of thetransistor 111 by temporally setting the gate of the transistor 111 in afloating state, as described above. In this case, as illustrated in FIG.1C, a capacitor 115 may be provided between the gate of the transistor111 and the one of the source and the drain of the transistor 111. Withprovision of the capacitor 115, bootstrap operation can be performedstably. Note that in the case where sufficient parasitic capacitance isgenerated between the gate of the transistor 111 and the one of thesource and the drain of the transistor 111, bootstrap operation can beperformed without provision of the capacitor 115.

In addition, although the structure where the direction of currentflowing through the transistor 112 which continuously outputs L-levelsignals is changed is given as an example in this embodiment, thedirection of current flowing through the transistor 111 may be changedin the case where the transistor 111 continuously outputs L-levelsignals. In this case, a signal whose potential is switched between thefirst potential V_(LH) and the second potential V_(LL) (V_(LH)>V_(LL))every given period can be used as the signal which is input to thewiring 101 (IN1).

Alternatively, in the case where the transistor 112 (or the transistor111) continuously outputs H-level signals, the direction of currentflowing through the transistor 112 (or the transistor 111) may bechanged. In this case, a signal whose potential is switched between afirst potential V_(HH) and a second potential V_(HL) (V_(HH)>V_(HL))every given period can be used as the signal which is input to thewiring 102 (or the wiring 101).

Needless to say, a structure where the direction of current is changedin both the transistor 111 and the transistor 112 may be used. Forexample, in the case where an H-level signal is supplied from the wiring101 to the wiring 103 through the transistor 111 and an L-level signalis supplied from the wiring 102 to the wiring 103 through the transistor112, a signal whose potential is switched between the first potentialV_(HH) and the second potential V_(HL) every given period can be used asthe signal which is input to the wiring 101 (IN1), and a signal whosepotential is switched between the first potential V_(LH) and the secondpotential V_(LL) every given period can be used as the signal which isinput to the wiring 102 (IN2).

Further, although n-channel transistors are used as the transistors 111,112, and 121 in this embodiment, p-channel transistors may be used (seeFIGS. 4A to 4F). In the case where p-channel transistors are used, byusing a signal whose potential is switched between the first potentialV_(HH) and the second potential V_(HL) every given period as the signalwhich is input to the wiring 102, operation can be performed such thatthe direction of current flowing through the transistor 112 is changed.Accordingly, deterioration in the transistor 112 is suppressed, so thata malfunction of the circuit can be suppressed. Note that althoughp-channel transistors are used as the transistors 111, 112, and 121 inFIGS. 4A to 4F, an n-channel transistor may be used as the transistor121.

Note that the structure described in this embodiment can be combinedwith a different structure described in this specification (including astructure described in any of the other embodiments) as appropriate.

Embodiment 2

In this embodiment, examples of a semiconductor device having astructure which is different from the structure in the above embodimentare described with reference to drawings.

The semiconductor device described in this embodiment includes at leastthe transistor 111 provided between the wiring 101 and the wiring 103and a plurality of transistors 112 a and 112 b provided in parallel witheach other between the wiring 102 and the wiring 103 (see FIG. SA). Thatis, the structure illustrated in FIG. 5A is a structure obtained byadding the transistor 112 b to the structure illustrated in FIG. 1A (thetransistor 112 a in FIGS. 5A to 5C corresponds to the transistor 112 inFIGS. 1A to 1C). Note that although FIG. 5A illustrates the case wheretwo transistors (the transistors 112 a and 112 b) are provided inparallel, three or more transistors may be provided.

One of a source and a drain of each of the transistors 112 a and 112 bis electrically connected to the wiring 102, and the other of the sourceand the drain of each of the transistors 112 a and 112 b is electricallyconnected to the wiring 103. The transistors 112 a and 112 b areprovided in parallel with each other. Therefore, by turning on at leastone of the transistors 112 a and 112 b, the signal which is input to thewiring 102 (IN2) is supplied to the wiring 103.

That is, a first signal corresponding to the signal which is input tothe wiring 101 (IN1) or a second signal corresponding to the signalwhich is input to the wiring 102 (IN2) is supplied to the wiring 103.

Note that although n-channel transistors are used as the transistors111, 112 a, and 112 b in FIGS. 5A to 5C, p-channel transistors may beused or CMOSs may be used. Further, the transistor III serves as aswitch provided between the wiring 101 and the wiring 103, and each ofthe transistors 112 a and 112 b serves as a switch provided between thewiring 102 and the wiring 103 (see FIG. 5B).

In this embodiment, a plurality of transistors provided in parallel (thetransistors 112 a and 112 b in FIGS. 5A to 5C) are alternately turned onand off. In addition, a structure where the direction of current flowingthrough the plurality of transistors is changed (a structure where thelevel of voltage applied to a terminal serving as the source or thedrain of each transistor is changed every period (the source and thedrain are switched)) is used. That is, on/off of the plurality oftransistors provided in parallel is controlled. Further, by controllingthe direction of current flowing through the plurality of transistors,concentration of an electric field on a channel portion (an end of thedrain) of each transistor is relieved, so that deterioration issuppressed.

A specific operating method is described below with reference todrawings.

[Operation in the Case where the Cycle of IN2 is Shorter than the Cycleof IN4 or IN5]

FIGS. 6A to 6F illustrate a signal which is output from the wiring 103(Out), the signal which is input to the wiring 101 (IN1), the signalwhich is input to the wiring 102 (IN2), the signal which is input to thegate of the transistor 111 (IN3), a signal which is input to a gate ofthe transistor 112 a (IN4), and a signal which is input to a gate of thetransistor 112 b (IN5), respectively. FIGS. 6C, 6E, and 6F illustratethe case where the cycle of the signal which is input to the wiring 102(IN2) is half of the cycle of the signal which is input to the gate ofthe transistor 112 a or 112 b (IN4 or IN5). Needless to say, signals tobe input (IN1 to IN5) are just examples, and signals are not limited tothe signals illustrated in FIGS. 6A to 6F.

In addition, FIG. 6G illustrates the direction of current flowingthrough the transistor 112 a and the transistor 112 b, and A₁, A₂, B₁,and B₂ correspond to the directions illustrated in FIGS. 5A to 5C.Further, a period during which the transistor is off and current doesnot flow is indicated by x.

First, in the period T1, the signal for turning on the transistor 111(IN3) is input to the gate of the transistor 111. Accordingly, thetransistor III is turned on, and the first signal corresponding to thesignal which is input to the wiring 101 (IN1) (here, an H-level signal(a selection signal)) is supplied to the wiring 103 through thetransistor 111. In the case where the gate of the transistor 121 isconnected to the wiring 103 (see FIG. 5C), the selection signal is inputto the gate of the transistor 121 which is connected to the wiring 103,so that the transistor 121 is turned on.

In the period T1, in the case where a potential of the signal which isinput to the gate of the transistor 111 (IN3) is V_(H), when it isassumed that the threshold voltage of the transistor 111 is Vth, apotential of a signal which is output to the wiring 103 is V_(N)−Vth. Inthis case, in order to set the potential of the signal which is outputto the wiring 103 at V_(H), the gate of the transistor 111 is set to bein a floating state in the period T1 and bootstrap operation isperformed. Needless to say, in order to set the potential of the signalwhich is output to the wiring 103 at V_(H), the potential of the signalwhich is input to the gate of the transistor 111 (IN3) may be set atV_(H)+Vth or higher in advance.

In addition, in the period T1, the transistors 112 a and 112 b are off.Note that this embodiment is not limited to this, and the transistors112 a and 112 b may be on as long as the selection signal is output tothe wiring 103. In this case, a potential of the signal which is inputto the wiring 102 (IN2) is preferably Vu.

Next, in the period T2, the signal for turning on the transistor 112 a(IN4) is input to the gate of the transistor 112 a. In this case, in thetransistor 112 a, a potential of a terminal which is connected to thewiring 102 (in this case, V_(LL)) is lower than a potential of aterminal which is connected to the wiring 103 (in this case, V_(H)), sothat the terminal which is connected to the wiring 102 is the source andthe terminal which is connected to the wiring 103 is the drain.Accordingly, a gate-source potential of the transistor 112 a(VgsB=V_(H)−V_(LL)) is higher than the threshold voltage of thetransistor 112 a, so that the transistor 112 a is turned on. Thus, asecond signal which corresponds to the signal input to the wiring 102(IN2) and has the potential V_(LL) (a non-selection signal) is suppliedto the wiring 103 through the transistor 112 a.

In the case where the gate of the transistor 121 is connected to thewiring 103, the non-selection signal is input to the gate of thetransistor 121 which is connected to the wiring 103, so that thetransistor 121 is turned off.

Subsequently, in the latter half of the period T2, the potential of thesignal which is input to the wiring 102 (IN2) is changed (is changedfrom V_(LL) to V_(LH) here). In this case, in the transistor 112 a, thepotential of the terminal which is connected to the wiring 102 (in thiscase, V_(LH)) is higher than the potential of the terminal which isconnected to the wiring 103 (in this case, V_(LL)), so that the terminalwhich is connected to the wiring 102 is the drain and the terminal whichis connected to the wiring 103 is the source. Accordingly, thegate-source potential of the transistor 112 a (VgsA=V_(H)−V_(LL)) ishigher than the threshold voltage of the transistor 112 a, so that thetransistor 112 a is kept on. Thus, the second signal which correspondsto the signal input to the wiring 102 (IN2) and has the potential V_(LH)(the non-selection signal) is supplied to the wiring 103 through thetransistor 112 a.

In the case where the gate of the transistor 121 is connected to thewiring 103, the non-selection signal is input to the gate of thetransistor 121 which is connected to the wiring 103, so that thetransistor 121 is kept off.

In this manner, in the first half of the period T2, in the transistor112 a, the potential of the terminal which is connected to the wiring102 is lower than the potential of the terminal which is connected tothe wiring 103, so that the terminal which is connected to the wiring102 is the source and the terminal which is connected to the wiring 103is the drain. Current flows from the drain to the source (in a directionB₁ in FIGS. 5A to 5C). On the other hand, in the latter half of theperiod T2, in the transistor 112 a, the potential of the terminal whichis connected to the wiring 102 is higher than the potential of theterminal which is connected to the wiring 103, so that the terminalwhich is connected to the wiring 102 is the drain and the terminal whichis connected to the wiring 103 is the source. Current flows from thedrain to the source (in a direction A₁ in FIGS. 5A to 5C).

Next, in the period T3, the transistor 112 a is turned off, and thesignal for turning on the transistor 112 b (IN5) is input to the gate ofthe transistor 112 b. In this case, in the transistor 112 b, a potentialof a terminal which is connected to the wiring 102 (in this case,V_(LL)) is lower than a potential of a terminal which is connected tothe wiring 103 (in this case, V_(LH)), so that the terminal which isconnected to the wiring 102 is the source and the terminal which isconnected to the wiring 103 is the drain. Accordingly, a gate-sourcepotential of the transistor 112 b (VgsB=V_(H)−V_(LL)) is higher than thethreshold voltage of the transistor 112 b, so that the transistor 112 bis turned on. Thus, the second signal which corresponds to the signalinput to the wiring 102 (IN2) and has the potential V_(LL) (thenon-selection signal) is supplied to the wiring 103 through thetransistor 112 b.

In the case where the gate of the transistor 121 is connected to thewiring 103, the non-selection signal is input to the gate of thetransistor 121 which is connected to the wiring 103, so that thetransistor 121 is kept off.

Subsequently, in the latter half of the period T3, the potential of thesignal which is input to the wiring 102 (IN2) is changed (is changedfrom V_(LL) to V_(LH), here). In this case, in the transistor 112 b, thepotential of the terminal which is connected to the wiring 102 (in thiscase, V_(LH)) is higher than the potential of the terminal which isconnected to the wiring 103 (in this case, V_(LL)), so that the terminalwhich is connected to the wiring 102 is the drain of the transistor 112b and the terminal which is connected to the wiring 103 is the source.Accordingly, the gate-source potential of the transistor 112 b(VgsA=V_(H)−V_(LL)) is higher than the threshold voltage of thetransistor 112 b, so that the transistor 112 b is kept on. Thus, thesecond signal which corresponds to the signal input to the wiring 102(IN2) and has the potential V_(LH) (the non-selection signal) issupplied to the wiring 103 through the transistor 112 b.

In the case where the gate of the transistor 121 is connected to thewiring 103, the non-selection signal is input to the gate of thetransistor 121 which is connected to the wiring 103, so that thetransistor 121 is kept off.

In this manner, in the first half of the period T3, in the transistor112 b, the potential of the terminal which is connected to the wiring102 is lower than the potential of the terminal which is connected tothe wiring 103, so that the terminal which is connected to the wiring102 is the source and the terminal which is connected to the wiring 103is the drain. Current flows from the drain to the source (in a directionB₂ in FIGS. 5A to 5C). On the other hand, in the latter half of theperiod T3, in the transistor 112 b, the potential of the terminal whichis connected to the wiring 102 is higher than the potential of theterminal which is connected to the wiring 103, so that the terminalwhich is connected to the wiring 102 is the drain and the terminal whichis connected to the wiring 103 is the source. Current flows from thedrain to the source (in a direction A₂ in FIGS. 5A to 5C).

In consecutive periods T4 to Tn, operation which is similar to theoperation in the period T2 or the period T3 is performed.

In the periods T3 to Tn, when the transistor 112 a is on, the secondsignal having the potential V_(LL) is supplied from the wiring 102 tothe wiring 103 in the first half of a period during which the transistor112 a is on, and the second signal having the potential V_(LH) issupplied from the wiring 102 to the wiring 103 in the latter half of theperiod during which the transistor 112 a is on. Therefore, in the firsthalf of the period during which the transistor 112 a is on, the terminalwhich is connected to the wiring 102 is the source, the terminal whichis connected to the wiring 103 is the drain, and current flows from thedrain to the source (in the direction B₁ in FIGS. 5A to 5C). Inaddition, in the latter half of the period during which the transistor112 a is on, the terminal which is connected to the wiring 102 is thedrain, the terminal which is connected to the wiring 103 is the source,and current flows from the drain to the source (in the direction A_(t)in FIGS. 5A to 5C).

Further, in the latter half of the period during which the transistor112 a is on (a period during which the transistor 112 b is off), apotential of the wiring 102 is V_(LH), so that the level of thegate-source voltage (Vgs) of the transistor 112 b is negative (Vgs<0 V).By providing a period during which the level of the gate-source voltage(Vgs) of the transistor 112 b is negative (Vgs<0 V) in this manner,deterioration in the transistor can be effectively suppressed.

In the periods T3 to Tn, when the transistor 112 b is on, the secondsignal having the potential V_(LL) is supplied from the wiring 102 tothe wiring 103 in the first half of a period during which the transistor112 b is on, and the second signal having the potential V_(LH) issupplied from the wiring 102 to the wiring 103 in the latter half of theperiod during which the transistor 112 b is on. Therefore, in the firsthalf of the period during which the transistor 112 b is on, the terminalwhich is connected to the wiring 102 is the source, the terminal whichis connected to the wiring 103 is the drain, and current flows from thedrain to the source (in the direction B₂ in FIGS. 5A to 5C). Inaddition, in the latter half of the period during which the transistor112 b is on, the terminal which is connected to the wiring 102 is thedrain, the terminal which is connected to the wiring 103 is the source,and current flows from the drain to the source (in the direction A₂ inFIGS. 5A to 5C).

Further, in the latter half of the period during which the transistor112 b is on (a period during which the transistor 112 a is off), thepotential of the wiring 102 is V_(LH), so that the level of thegate-source voltage (Vgs) of the transistor 112 a is negative (Vgs<0 V).By providing a period during which the level of the gate-source voltage(Vgs) of the transistor 112 a is negative (Vgs<0 V) in this manner,deterioration in the transistor can be effectively suppressed.

With a structure in which a plurality of transistors provided inparallel are alternately turned on and off and the direction of currentflowing through the transistor is changed (the level of voltage appliedto a first terminal and a second terminal (terminals serving as a sourceand a drain) of the transistor is changed every period (the source andthe drain are switched)) in a period during which the transistor is onin this manner, concentration of an electric field on a channel portion(an end of the drain) of the transistor is relieved. Thus, deteriorationin the transistor can be suppressed. Accordingly, a malfunction of acircuit due to deterioration in the transistor is suppressed, so thatreliability can be improved.

Further, as illustrated in FIGS. 6A to 6G, it is preferable that thelength of the period during which the transistor 112 a or 112 b is onand the length of the period during which the transistor 112 a or 112 bis off be the same or substantially the same in view of controllability.In this case, the direction of current flowing through the transistorcan be changed every half of the period during which the transistor 112a or 112 b is on.

Note that although FIGS. 6A to 6G illustrate the case where theplurality of transistors provided in parallel (the transistors 112 a and112 b) are alternately turned on and off, the period during which thetransistor 112 a is on and the period during which the transistor 112 bis on may partly overlap with each other, or the period during which thetransistor 112 a is off and the period during which the transistor 112 bis off may partly overlap with each other. That is, a period duringwhich both the transistors 112 a and 112 b are on or a period duringwhich both the transistors 112 a and 112 b are off may be provided.

Although FIGS. 6C, 6E, and 6F illustrate the case where the cycle of thesignal which is input to the wiring 102 (IN2) is half of the cycle ofthe signal which is input to the gate of the transistor 112 a or 112 b(IN4 or IN5), this embodiment is not limited to this. The cycle may beshorter or longer than half of the cycle of the signal which is input tothe gate of the transistor 112 a or 112 b (IN4 or IN5). In addition, inFIG. 6C, the phase of the signal which is input to the wiring 102 (IN2)may be changed. For example, the phase of the signal which is input tothe wiring 102 (IN2) may be changed by half or quarter of the cycle.

Further, in the operation of the structures illustrated in FIGS. 5A to5C, the cycle of the signal which is input to the wiring 102 (IN2) isnot limited to a cycle which is shorter than the cycle of the signalwhich is input to the gate of the transistor 112 a or 112 b (IN4 orIN5). The case where the cycle of the signal which is input to thewiring 102 (IN2) is equal to the cycle of the signal which is input tothe gate of the transistor 112 a or 112 b (IN4 or IN5) and the casewhere the cycle of the signal which is input to the wiring 102 (IN2) islonger than the cycle of the signal which is input to the gate of thetransistor 112 a or 112 b (IN4 or IN5) are described below withreference to drawings.

[Operation in the Case where the Cycle of IN2 is Longer than the Cycleof IN4 or IN5]

In the following description, FIGS. 7A to 7F and FIGS. 8A to 8Fillustrate the signal which is output from the wiring 103 (Out), thesignal which is input to the wiring 101 (IN1), the signal which is inputto the wiring 102 (IN2), the signal which is input to the gate of thetransistor 111 (IN3), the signal which is input to the gate of thetransistor 112 a (IN4), and the signal which is input to the gate of thetransistor 112 b (IN5). FIGS. 7C, 7E and 7F and FIGS. 8C, 8E, and 8Fillustrate the case where the cycle of the signal which is input to thewiring 102 (IN2) is longer than the cycle of the signal which is inputto the gate of the transistor 112 a or 112 b (IN4 or IN5) (the casewhere the cycle of IN2 is twice the cycle of IN4 or IN5). Needless tosay, signals to be input (IN1 to IN5) are just examples, and signals arenot limited to the signals illustrated in FIGS. 7A to 7F and FIGS. 8A to8F.

In addition, FIG. 7G and FIG. 8G illustrate the direction of currentflowing through the transistor 112 a and the transistor 112 b, and A₁,A₂, B₁, and B₂ correspond to the directions illustrated in FIGS. 5A to5C. Further, a period during which the transistor is off and currentdoes not flow is indicated by x. Furthermore, a period during which thetransistor is on but current does not flow is indicated by -.

First, in the period T1, the signal for turning on the transistor 111(IN3) is input to the gate of the transistor 111. Here, operation whichis similar to the operation in the period T1 in FIGS. 6A to 6G isperformed.

Next, in the period T2, the signal for turning on the transistor 112 a(IN4) is input to the gate of the transistor 112 a. In this case, in thetransistor 112 a, the potential of the terminal which is connected tothe wiring 102 (in this case, V_(LL)) is lower than the potential of theterminal which is connected to the wiring 103 (in this case, V_(H)), sothat the terminal which is connected to the wiring 102 is the source andthe terminal which is connected to the wiring 103 is the drain.Accordingly, the gate-source potential of the transistor 112 a(VgsB=V_(H)−V_(LL)) is higher than the threshold voltage of thetransistor 112 a, so that the transistor 112 a is turned on. Thus, thesecond signal which corresponds to the signal input to the wiring 102(IN2) and has the potential V_(LL) (the non-selection signal) issupplied to the wiring 103 through the transistor 112 a.

In the case where the gate of the transistor 121 is connected to thewiring 103 (see FIG. 5C), the non-selection signal is input to the gateof the transistor 121 which is connected to the wiring 103, so that thetransistor 121 is turned off.

In this manner, in the period T2, in the transistor 112 a, the potentialof the terminal which is connected to the wiring 102 is lower than thepotential of the terminal which is connected to the wiring 103, so thatthe terminal which is connected to the wiring 102 is the source and theterminal which is connected to the wiring 103 is the drain. Currentflows from the drain to the source (in the direction B₁ in FIGS. 5A to5C).

Next, in the period T3, the signal for turning off the transistor 112 a(IN4) is input to the gate of the transistor 112 a, and the signal forturning on the transistor 112 b (IN5) is input to the gate of thetransistor 112 b, so that on/off of the transistors 112 a and 112 b isswitched. In this case, in the transistor 112 b, the potential of theterminal which is connected to the wiring 102 (in this case, V_(LH)) ishigher than the potential of the terminal which is connected to thewiring 103 (in this case, V_(LL)), so that the terminal which isconnected to the wiring 102 is the drain and the terminal which isconnected to the wiring 103 is the source. Accordingly, the gate-sourcepotential of the transistor 112 b (VgsA=V_(H)−V_(LL)) is higher than thethreshold voltage of the transistor 112 b, so that the transistor 112 bis turned on. Thus, the second signal which corresponds to the signalinput to the wiring 102 (IN2) and has the potential V_(LH) (thenon-selection signal) is supplied to the wiring 103 through thetransistor 112 b.

In this manner, in the period T3, in the transistor 112 b, the potentialof the terminal which is connected to the wiring 103 is lower than thepotential of the terminal which is connected to the wiring 102, so thatthe terminal which is connected to the wiring 103 is the source and theterminal which is connected to the wiring 102 is the drain. Currentflows from the drain to the source (in the direction A₂ in FIGS. 5A to5C).

Further, in the period T3, the level of the gate-source voltage (Vgs) ofthe transistor 112 a is negative (Vgs<0 V). By providing a period duringwhich the level of the gate-source voltage (Vgs) of the transistor 112 ais negative (Vgs<0 V) in this manner, deterioration in the transistorcan be more effectively suppressed as compared to the case where Vgs=0V.

Next, in the period T4, the signal for turning on the transistor 112 a(IN4) is input to the gate of the transistor 112 a, and the signal forturning off the transistor 112 b (IN5) is input to the gate of thetransistor 112 b, so that on/off of the transistors 112 a and 112 b isswitched. In addition, since the potential of the wiring 102 is kept atV_(LH), the potential of the wiring 103 is also kept at V_(LH).Therefore, in the transistor 112 a, the potential of the terminal whichis connected to the wiring 102 is equal to the potential of the terminalwhich is connected to the wiring 103, so that current does not flowthrough the transistor 112 a.

Next, in the period T5, the signal for turning off the transistor 112 a(IN4) is input to the gate of the transistor 112 a, and the signal forturning on the transistor 112 b (IN5) is input to the gate of thetransistor 112 b, so that on/off of the transistors 112 a and 112 b isswitched. In this case, in the transistor 112 b, the potential of theterminal which is connected to the wiring 102 (in this case, V_(LL)) islower than the potential of the terminal which is connected to thewiring 103 (in this case, V_(LH)), so that the terminal which isconnected to the wiring 102 is the source and the terminal which isconnected to the wiring 103 is the drain. Accordingly, the gate-sourcepotential of the transistor 112 b (VgsB=V_(H)−V_(LL)) is higher than thethreshold voltage of the transistor 112 b, so that the transistor 112 bis turned on. Thus, the second signal which corresponds to the signalinput to the wiring 102 (IN2) and has the potential V_(LL) (thenon-selection signal) is supplied to the wiring 103 through thetransistor 112 b.

In this manner, in the period T5, in the transistor 112 b, the potentialof the terminal which is connected to the wiring 102 is lower than thepotential of the terminal which is connected to the wiring 103, so thatthe terminal which is connected to the wiring 102 is the source and theterminal which is connected to the wiring 103 is the drain. Currentflows from the drain to the source (in the direction B₂ in FIGS. 5A to5C).

In consecutive periods T6 to Tn, the operation in the periods T2 to T5is repeated. Thus, the transistor 111 is kept off, the transistors 112 aand 112 b are alternately turned on, and a signal whose potential isswitched between the potential V_(LH) and the potential V_(LL) everygiven period is input to the wiring 103. Therefore, in the case wherethe gate of the transistor 121 is connected to the wiring 103, thetransistor 121 is stably kept off.

Note that in the operating method illustrated in FIGS. 7A to 7G, it isimpossible to change the direction of current flowing through thetransistor 112 a (to switch the source and the drain). Therefore, it ispreferable to use a structure where the cycle of the signal which isinput to the wiring 102 (IN2) is changed every certain period and thedirection of current flowing through the transistor 112 a is changed.

FIGS. 8A to 8G illustrate the case where the cycle of the signal whichis input to the wiring 102 (IN2) is changed and the direction of currentflowing through the transistor 112 a is changed. In the operationillustrated in FIGS. 8A to 8G, it is impossible to change the directionof current flowing through the transistor 112 b (to switch the sourceand the drain). Therefore, in operating the circuit, by switching theoperation illustrated in FIGS. 7A to 7G and the operation illustrated inFIGS. 8A to 8G, deterioration in the transistor 112 a and the transistor112 b can be suppressed even in the case where the cycle of the signalwhich is input to the wiring 102 (IN2) is made longer than the cycle ofthe signal which is input to the transistor 112 a or 112 b (IN4 or IN5).

Note that although FIGS. 7A to 7G and FIGS. 8A to 8G illustrate the casewhere the plurality of transistors provided in parallel (the transistors112 a and 112 b) are alternately turned on and off, the period duringwhich the transistor 112 a is on and the period during which thetransistor 112 b is on may partly overlap with each other, or the periodduring which the transistor 112 a is off and the period during which thetransistor 112 b is off may partly overlap with each other. That is, aperiod during which both the transistors 112 a and 112 b are on or aperiod during which both the transistors 112 a and 112 b are off may beprovided.

Although FIGS. 7C, 7E, and 7F and FIGS. 8C, 8E, and 8F illustrate thecase where the cycle of the signal which is input to the wiring 102(IN2) is twice the cycle of the signal which is input to the gate of thetransistor 112 a or 112 b (IN4 or IN5), this embodiment is not limitedto this. The cycle may be shorter or longer than twice the cycle of thesignal which is input to the gate of the transistor 112 a or 112 b (IN4or IN5).

[Operation in the Case where the Cycle of IN2 is Equal to the Cycle ofIN4 or IN5]

In the following description, FIGS. 9A to 9F and FIGS. 10A to 10Fillustrate the signal which is output from the wiring 103 (Out), thesignal which is input to the wiring 101 (IN1), the signal which is inputto the wiring 102 (IN2), the signal which is input to the gate of thetransistor 111 (IN3), the signal which is input to the gate of thetransistor 112 a (IN4), and the signal which is input to the gate of thetransistor 112 b (IN5). FIGS. 9C, 9E, and 9F and FIGS. 10C, 10E, and 10Fillustrate the case where the cycle of the signal which is input to thewiring 102 (IN2) is equal to the cycle of the signal which is input tothe gate of the transistor 112 a or 112 b (IN4 or IN5). Needless to say,signals to be input (IN1 to IN5) are just examples, and signals are notlimited to the signals illustrated in FIGS. 9A to 9F and FIGS. 10A to10F.

In addition, FIG. 9G and FIG. 10G illustrate the direction of currentflowing through the transistor 112 a and the transistor 112 b, and A₁,A₂, B₁, and B₂ correspond to the directions illustrated in FIGS. 5A to5C. Further, a period during which the transistor is off and currentdoes not flow is indicated by x. Furthermore, a period during which thetransistor is on but current does not flow is indicated by -.

In this embodiment, the transistors are operated by alternatelyperforming the operation illustrated in FIGS. 9A to 9G and the operationillustrated in FIGS. 10A to 100 every given period.

First, in periods Tx1 to Txn, the second signal having the potentialV_(LL) is supplied from the wiring 102 to the wiring 103 when thetransistor 112 a is on, and the second signal having the potentialV_(LH) is supplied from the wiring 102 to the wiring 103 when thetransistor 112 b is on (see FIGS. 9A to 9G).

Therefore, in the periods Tx1 to Txn, when the transistor 112 a is on,in the transistor 112 a, the terminal which is connected to the wiring102 is the source, the terminal which is connected to the wiring 103 isthe drain, and current flows from the drain to the source (in thedirection B in FIGS. 5A to 5C). In addition, when the transistor 112 bis on, in the transistor 112 b, the terminal which is connected to thewiring 103 is the source, the terminal which is connected to the wiring102 is the drain, and current flows from the drain to the source (in thedirection A₂ in FIGS. 5A to 5C). Further, in the case where thetransistor 112 a is off, the level of the gate-source voltage (Vgs) ofthe transistor 112 a can be negative (Vgs<0 V). Thus, deterioration inthe transistor can be more effectively suppressed as compared to thecase where Vgs=0 V.

Note that in each of the transistor 112 a and the transistor 112 b, inthe case where current flows and the potential of the terminal which isconnected to the wiring 102 is equal to the potential of the terminalwhich is connected to the wiring 103, them is no distinction between thesource and the drain.

First, in periods Ty1 to Tyn, the second signal having the potentialV_(LH) is supplied from the wiring 102 to the wiring 103 when thetransistor 112 a is on, and the second signal having the potentialV_(LL) is supplied from the wiring 102 to the wiring 103 when thetransistor 112 b is on (see FIGS. 10A to 10G).

Therefore, in the periods Ty1 to Tyn, when the transistor 112 a is on,in the transistor 112 a, the terminal which is connected to the wiring102 is the drain, the terminal which is connected to the wiring 103 isthe source, and current flows from the drain to the source (in thedirection A₁ in FIGS. 5A to 5C). In addition, when the transistor 112 bis on, in the transistor 112 b, the terminal which is connected to thewiring 103 is the drain, the terminal which is connected to the wiring102 is the source, and current flows from the drain to the source (inthe direction B₂ in FIGS. 5A to 5C). Further, in the case where thetransistor 112 b is off the level of the gate-source voltage (Vgs) ofthe transistor 112 b can be negative (Vgs<0 V). Thus, deterioration inthe transistor can be more effectively suppressed as compared to thecase where Vgs=0 V.

Therefore, by changing (for example, inverting) the cycle of the signalwhich is input to the wiring 102 (IN2) every given period and bycombining the operation in FIGS. 9A to 9G with the operation in FIGS.10A to 10G (for example, see FIGS. 11A to 11G), a structure can be usedin which the direction of current flowing through the transistors 112 aand 112 b is changed (the level of voltage applied to the terminalsserving as the source and the drain of the transistor is changed everyperiod (the source and the drain are switched)). Accordingly,concentration of an electric field on channel portions (ends of thedrains) of the transistors 112 a and 112 b is relieved. Thus,deterioration can be suppressed. Further, by alternately turning on andoff the plurality of transistors provided in parallel (the transistors112 a and 112 b), deterioration in the transistors can be suppressed.

In a given period, for example, in the case where the semiconductordevice in this embodiment is used as a gate driver of a display device,the operation illustrated in FIGS. 9A to 9G and the operationillustrated in FIGS. 10A to 10G can be switched every one frame period.

Note that although FIGS. 9A to 9G and FIGS. 10A to 10G illustrate thecase where rising and falling of the signal which is input to the wiring102 (IN2) and the signal which is input to the gate of the transistor112 a or 112 b (IN4 or IN5) are performed at the same timing, thisembodiment is not limited to this. For example, the operation may beperformed by changing the cycle of the signal which is input to thewiring 102 (IN2) by quarter of the cycle.

With a structure in which a plurality of transistors provided inparallel are alternately turned on and off and the direction of currentflowing through the plurality of transistors is changed (the level ofvoltage applied to terminals serving as a source and a drain of thetransistor is changed every period (the source and the drain areswitched)) as illustrated in this embodiment, concentration of anelectric field on a channel portion (an end of the drain) of thetransistor is relieved. Thus, deterioration in the transistor can beeffectively suppressed.

Note that although n-channel transistors are used as the transistors111, 112 a, 112 b, and 121 in this embodiment, p-channel transistors maybe used. Also in this case, by performing operation such that thedirection of current flowing through the transistors 112 a and 112 b ischanged, deterioration in the transistors is suppressed, so that amalfunction of the circuit can be suppressed.

Further, in this embodiment, a structure where L-level signals arecontinuously output in operating the circuit is used. However, in thecase where H-level signals are continuously output, a structure can beused in which a plurality of transistors are provided in parallel witheach other between the wiring 101 and the wiring 103 and a signal whosepotential is switched between the first potential V_(LH) and the secondpotential V_(LL) every given period is used as the signal which is inputto the wiring 101.

Note that the structure described in this embodiment can be combinedwith a different structure described in this specification (including astructure described in any of the other embodiments) as appropriate.

Embodiment 3

In this embodiment, examples of a semiconductor device having astructure which is different from the structure in the above embodimentare described with reference to drawings.

The semiconductor device described in this embodiment includes at leastthe transistor 111 provided between the wiring 101 and the wiring 103,the transistor 112 provided between the wiring 102 and the wiring 103,and a transistor 114 provided between a wiring 104 and the wiring 103(see FIG. 12A).

One of a source and a drain of the transistor 114 is electricallyconnected to the wiring 104, and the other of the source and the drainof the transistor 114 is electrically connected to the wiring 103. Thatis, the structures illustrated in FIGS. 12A to 12C are structuresobtained by adding the wiring 104 to the structures illustrated in FIGS.5A to 5C and by electrically connecting the one of the source and thedrain of the transistor 112 b in FIGS. 5A to 5C not to the wiring 102but to the wiring 104. Therefore, by turning on the transistor 114, asignal which is input to the wiring 104 (IN6) is supplied to the wiring103.

Therefore, a first signal corresponding to the signal which is input tothe wiring 101 (IN1), a second signal corresponding to the signal whichis input to the wiring 102 (IN2), or a third signal corresponding to thesignal which is input to the wiring 104 (IN6) is supplied to the wiring103.

By separately providing the wiring 102 and the wiring 104 in thismanner, different signals can be simultaneously supplied to thetransistor 112 and the transistor 114. Accordingly, the frequency ofsignals is lowered, so that power consumption can be reduced.

Note that although n-channel transistors are used as the transistors111, 112, and 114 in FIGS. 12A to 12C, p-channel transistors may be usedor CMOSs may be used. Further, the transistor 111 serves as a switchprovided between the wiring 101 and the wiring 103; the transistor 112serves as a switch provided between the wiring 102 and the wiring 103:the transistor 114 serves as a switch provided between the wiring 104and the wiring 103 (see FIG. 12B).

In this embodiment, in the case where certain signals (e.g.,non-selection signals) are continuously supplied to the wiring 103, astructure is used in which a plurality of transistors each having one ofa source and a drain which is connected to the wiring 103 and the otherof the source and the drain which is connected to a different wiring(the transistors 112 and 114 in FIGS. 12A to 12C) are alternately turnedon and off and the direction of current flowing through the plurality oftransistors is changed (the level of voltage applied to terminalsserving as the sources and the drains of the transistors is changedevery period (the sources and the drains are switched)). That is, bycontrolling on/off of the plurality of transistors and the direction ofcurrent flowing through the transistors, concentration of an electricfield on channel portions (ends of the drains) of the transistors isrelieved, so that deterioration is suppressed.

A specific operating method is described below with reference todrawings.

In the following description, FIGS. 13A to 13G and FIGS. 14A to 14Gillustrate the signal which is output from the wiring 103 (Out), thesignal which is input to the wiring 101 (IN1), the signal which is inputto the wiring 102 (IN2), the signal which is input to the wiring 104(IN6), the signal which is input to the gate of the transistor 111(IN3), the signal which is input to the gate of the transistor 112(IN4), and a signal which is input to a gate of the transistor 114(IN5). Needless to say, signals to be input (IN1 to IN6) are justexamples, and signals are not limited to the signals illustrated inFIGS. 13A to 13G and FIGS. 14A to 14G.

In addition, FIG. 13H and FIG. 14H illustrate the direction of currentflowing through the transistor 112 and the transistor 114, and A₁, A₂,B₁, and B₂ correspond to the directions illustrated in FIGS. 12A to 12C.Further, a period during which the transistor is off and current doesnot flow is indicated by x. Furthermore, a period during which thetransistor is on but current does not flow is indicated by -.

In FIGS. 13A to 13H and FIGS. 14A to 14H, the potential V_(LL) and thepotential V_(LH) are alternately applied to the wiring 102 and thewiring 104 every given period. In the following description, the casewhere the periods Tx1 to Txn during which the potential V_(LL) isapplied to the wiring 102 and the potential V_(LH) is applied to thewiring 104 and the periods Ty1 to Tyn during which the potential V_(LH)is applied to the wiring 102 and the potential V_(LL) is applied to thewiring 104 are switched every given period is described.

First, in the periods Tx1 to Txn, when the transistor 111 is on (in theperiod Tx1), the first signal corresponding to the signal which is inputto the wiring 101 (IN1) (here, an H-level signal (a selection signal))is supplied to the wiring 103. In the case where the gate of thetransistor 121 is connected to the wiring 103 (see FIG. 12C), theselection signal is input to the gate of the transistor 121 which isconnected to the wiring 103, so that the transistor 121 is turned on.

In addition, in the periods Tx1 to Txn, the second signal having thepotential V_(LL) is supplied from the wiring 102 to the wiring 103 whenthe transistor 112 is on (here, in the periods Tx2, Tx4, Tx6, Tx8, andTxn), and the third signal having the potential V_(LH) is supplied fromthe wiring 104 to the wiring 103 when the transistor 114 is on (here, inthe periods Tx3, Tx5, and Tx7) (see FIGS. 13A to 13H). In the case wherethe gate of the transistor 121 is connected to the wiring 103 (see FIG.12C), the transistor 121 is turned off.

Therefore, in the periods Tx1 to Txn, when the transistor 112 is on, inthe transistor 112, the terminal which is connected to the wiring 102 isthe source, the terminal which is connected to the wiring 103 is thedrain, and current flows from the drain to the source (in a direction B,in FIGS. 12A to 12C). In addition, when the transistor 114 is on, in thetransistor 114, a terminal which is connected to the wiring 103 is thesource, a terminal which is connected to the wiring 102 is the drain,and current flows from the drain to the source (in a direction A₂ inFIGS. 12A to 12C).

In the periods Ty1 to Tyn, the second signal having the potential V_(LH)is supplied from the wiring 102 to the wiring 103 when the transistor112 is on (here, in the periods Ty4, Ty6, Ty8, and Tyn), and the thirdsignal having the potential V_(LL) is supplied from the wiring 104 tothe wiring 103 when the transistor 114 is on (here, in the periods Ty3,Ty5, and Ty7) (see FIGS. 14A to 14H).

Therefore, in the periods Ty1 to Tyn, when the transistor 112 is on, inthe transistor 112, the terminal which is connected to the wiring 102 isthe drain, the terminal which is connected to the wiring 103 is thesource, and current flows from the drain to the source (in a directionA₃ in FIGS. 12A to 12C). In addition, when the transistor 114 is on, inthe transistor 114, the terminal which is connected to the wiring 103 isthe drain, the terminal which is connected to the wiring 102 is thesource, and current flows from the drain to the source (in a directionB₂ in FIGS. 12A to 12C).

Therefore, by inverting the signal which is input to the wiring 102(IN2) and the signal which is input to the wiring 104 (IN6) every givenperiod and by combining the operation in FIGS. 13A to 131H with theoperation in FIGS. 14A to 14H (for example, see FIGS. 15A to 15H), astructure can be used in which the direction of current flowing throughthe transistors 112 and 114 is changed (the level of voltage applied tothe terminals serving as the source and the drain of the transistor ischanged every period (the source and the drain are switched)).Accordingly, concentration of an electric field on channel portions(ends of the drains) of the transistors 112 and 114 is relieved. Thus,deterioration can be suppressed. Further, by alternately turning on andoff the plurality of transistors (the transistors 112 and 114),deterioration in the transistors can be suppressed.

In a given period, for example, in the case where the semiconductordevice in this embodiment is used as a gate driver of a display device,the operation illustrated in FIGS. 13A to 13H and the operationillustrated in FIGS. 14A to 14H can be switched every one frame period.

Further, with the structure described in this embodiment, even in thecase where certain signals having a plurality of potentials (e.g.,non-selection signals) are continuously supplied to the wiring 103,potentials of the wirings 102 and 104 can be made constant. Thus, powerconsumption can be reduced.

Note that the structure described in this embodiment can be combinedwith a different structure described in this specification (including astructure described in any of the other embodiments) as appropriate.

Embodiment 4

In this embodiment, applications of the structures described in theabove embodiments are described with reference to drawings.

Any of the structures described in the above embodiments can be used fora scan line driver circuit and/or a signal line driver circuit (see FIG.16). A display device includes the scan line driver circuit (gatedriver), the signal line driver circuit (source driver), and a pixelportion.

FIG. 17 illustrates the case where the structure described in Embodiment1 is used for a gate driver of a display device. In specific, FIG. 17illustrates the case where the structure described in Embodiment 1 isused for a gate driver of a liquid crystal display device including aplurality of pixels.

In each of the transistors 121 provided in the plurality of pixels, thegate is electrically connected to any one of wirings 103 a to 103 cserving as gate lines; the one of the source and the drain iselectrically connected to any one of wirings 141 a to 141 c serving assource lines; the other of the source and the drain is electricallyconnected to a pixel electrode 125. In addition, a liquid crystalmaterial is provided between the pixel electrode 123 and a counterelectrode 126. Note that although wirings 101 a to 101 c areelectrically isolated from each other in FIG. 17, the wirings 101 a to101 c may be provided in common.

In addition, in order to turn on the transistors 121, H-level signalswhich are selection signals are supplied from the wirings 101 a to 101 cthrough the transistors 111. In order to turn off the transistors 121,L-level signals which are non-selection signals are supplied from thewirings 102 through the transistors 112.

In the case of a method by which data is written by selecting pixels ineach row, such as a line-at-a-time driving method, it is necessary tosupply a non-selection signal for turning off the transistor 121provided in each pixel to the gate of the transistor 121 after thetransistor 121 is turned on. In particular, it is effective tocontinuously supply non-selection signals to the wirings 103 a to 103 cthrough the transistors 112 for a predetermined period in turning offthe transistors 121 stably.

Here, by using a structure where a plurality of L-level signals whichare non-selection signals are input to the wirings 102 and the directionof current flowing through the transistors 112 is changed in a periodduring which the transistors 112 are on as described in Embodiment 1,deterioration in the transistors 112 can be suppressed. Accordingly, thetransistor 121 provided in each pixel can be stably turned off and amalfunction of a circuit due to deterioration in the transistors 112 canbe suppressed.

In particular, in the case where amorphous silicon or microcrystallinesilicon is used for a channel formation region of a transistor, it iseffective to suppress deterioration in a transistor which is kept on fora long time in an operating period of a circuit in suppressing amalfunction of the circuit.

In addition, although n-channel transistors are used as all thetransistors 111 and 112 provided in the gate driver and the transistors121 provided in the pixels in FIG. 17, p-channel transistors may be usedor CMOSs can be used.

Further, although FIG. 17 illustrates the case where the structuredescribed in Embodiment 1 is used for the gate driver, this embodimentis not limited to this. The structure described in Embodiment 2 or 3 canbe used.

In the case where the structure described in Embodiment 2 is used, inFIG. 17, the transistor 112 b may be provided (see FIG. 18). The one ofthe source and the drain of the transistor 112 b is electricallyconnected to the wiring 102. The other of the source and the drain ofthe transistor 112 b is connected to the wiring 103 a (or the wiring 103b or 103 c). The transistor 112 b is provided in parallel with thetransistor 112.

In the case where the structure described in Embodiment 3 is used, inFIG. 17, the wiring 104 may be additionally provided and the transistor114 may be provided (see FIG. 19). The one of the source and the drainof the transistor 114 is electrically connected to the wiring 104. Theother of the source and the drain of the transistor 114 is connected tothe wiring 103 a (or the wiring 103 b or 103 c).

Note that FIG. 19 illustrates the case where a structure in which theone of the source and the drain of the transistor 112 is electricallyconnected to the wiring 102 and the one of the source and the drain ofthe transistor 114 is electrically connected to the wiring 104 and astructure in which the one of the source and the drain of the transistor112 is electrically connected to the wiring 104 and the one of thesource and the drain of the transistor 114 is electrically connected tothe wiring 102 are alternately provided in a row direction; however,this embodiment is not limited to this. In all the rows, the one of thesource and the drain of the transistor 112 may be electrically connectedto the wiring 102 and the one of the source and the drain of thetransistor 114 may be electrically connected to the wiring 104, or theone of the source and the drain of the transistor 112 may beelectrically connected to the wiring 104 and the one of the source andthe drain of the transistor 114 may be electrically connected to thewiring 102.

Further, although FIG. 17, FIG. 18, and FIG. 19 illustrate the casewhere any of the structures described in the above embodiments is usedfor a liquid crystal display device, any of the structures described inthe above embodiments can be used for a gate driver and/or a sourcedriver of a different display device (e.g., an organic EL displaydevice). For example, as illustrated in FIG. 20, any of the structuresdescribed in the above embodiments can be used for a gate driver of anorganic EL display device including the transistor 121, a transistor128, and a light-emitting element 129 in a pixel.

In FIG. 20, in each of the transistors 121 provided in a plurality ofpixels, the gate is electrically connected to any one of the wirings 103a to 103 c, and the one of the source and the drain is electricallyconnected to a gate of the transistor 128. Note that each of thetransistors 121 is referred to as a switching transistor in some cases.Further, one of a source and a drain of the transistor 128 iselectrically connected to one of wirings 142 a and 142 b, and the otherof the source and the drain of the transistor 128 is electricallyconnected to one of electrodes of the light-emitting element 129. Notethat the transistor 128 is referred to as a driving transistor in somecases.

Note that the circuit structures illustrated in FIG. 17, FIG. 18, FIG.19, and FIG. 20 are just examples, and a different circuit structure canbe used.

Next, circuit structures which can be used for a driver circuit aredescribed.

A structure where one of a source and a drain of a different transistor(here, a transistor 161) is electrically connected to the gate of thetransistor 111 can be used (see FIG. 21A). Alternatively, a structurewhere one of a source and a drain of a different transistor (here, atransistor 162) is electrically connected to the gate of the transistor111 in the structure of FIG. 21A can be used (see FIG. 21B).Alternatively, a structure where one of a source and a drain of adifferent transistor (here, a transistor 163) is electrically connectedto the gate of the transistor 112 in the structure of FIG. 21A can beused (see FIG. 21C).

Note that in FIG. 21B, a wiring to which the other of the source and thedrain of the transistor 162 is connected may be electrically connectedto the wiring 102, or the gate of the transistor 112 and a gate of thetransistor 162 may be electrically connected to the same wiring.

Alternatively, in FIGS. 21A to 21C, a gate of the transistor 161 may beelectrically connected to the drain of the transistor 161 (thetransistor 161 may be diode-connected) (see FIGS. 22A to 22C).

Next, specific circuit structures in the case where any of thestructures described in the above embodiments is used for a gate driverare described in detail.

A structure example of a shift register included in the gate driver isdescribed with reference to FIG. 23. Note that FIG. 23 illustrates thestructure of the shift register in an n-th stage.

The shift register in the n-th stage includes first to seventhtransistors 201 to 207 and a capacitor 208. Note that although n-channeltransistors are used as the first to seventh transistors 201 to 207 inFIG. 23, p-channel transistors may be used.

A gate of the first transistor 201 is electrically connected to a sourceof the second transistor 202, one of a source and a drain of the thirdtransistor 203, one of a source and a drain of the fourth transistor204, and a gate of the fifth transistor 205. One of a source and a drainof the first transistor 201 is electrically connected to a wiring 212 a.The other of the source and the drain of the first transistor 201 iselectrically connected to a gate of the third transistor 203, a gate ofthe sixth transistor 206, and one of electrodes of the capacitor 208.

A gate of the second transistor 202 is electrically connected to a drainof the second transistor 202 (the second transistor 202 isdiode-connected). The source of the second transistor 202 iselectrically connected to the gate of the first transistor 201 and thegate of the fifth transistor 205. In addition, a signal which is outputfrom an (n−1)th stage is input to the drain of the second transistor202.

The gate of the third transistor 203 is electrically connected to theother of the source and the drain of the first transistor 201 and theone of the electrodes of the capacitor 208. The one of the source andthe drain of the third transistor 203 is electrically connected to thegate of the fifth transistor 205. The other of the source and the drainof the third transistor 203 is electrically connected to the wiring 212a.

A signal which is output from an (n+1)th stage is input to a gate of thefourth transistor 204. The one of the source and the drain of the fourthtransistor 204 is electrically connected to the gate of the fifthtransistor 205. The other of the source and the drain of the fourthtransistor 204 is electrically connected to the wiring 212 a.

The gate of the fifth transistor 205 is electrically connected to thegate of the first transistor 201, the source of the second transistor202, the one of the source and the drain of the third transistor 203,and the one of the source and the drain of the fourth transistor 204.One of a source and a drain of the fifth transistor 205 is electricallyconnected to a wiring 211. The other of the source and the drain of thefifth transistor 205 is electrically connected to a wiring 213.

The gate of the sixth transistor 206 is electrically connected to theother of the source and the drain of the first transistor 201 and theone of the electrodes of the capacitor 208. One of a source and a drainof the sixth transistor 206 is electrically connected to a wiring 212 b.The other of the source and the drain of the sixth transistor 206 iselectrically connected to the wiring 213.

An inverted clock signal (CLKB) is input to a gate of the seventhtransistor 207. One of a source and a drain of the seventh transistor207 is electrically connected to a wiring 212 c. The other of the sourceand the drain of the seventh transistor 207 is electrically connected tothe wiring 213.

The one of the electrodes of the capacitor 208 is electrically connectedto the gate of the third transistor 203 and the gate of the sixthtransistor 206. The other of the electrodes of the capacitor 208 iselectrically connected to the wiring 211.

A clock signal (CLK) is input to the wiring 211. L-level signals orL-level signals whose potentials are changed are input to the wirings212 a to 212 c. Note that signals which are input to the wirings are notlimited to these signals, and signals can be selected as appropriate.

In the structure illustrated in FIG. 23, in a period other than a periodduring which the gates in the n-th stage are selected (a non-selectionperiod during which non-selection signals are supplied to pixels), thethird transistor 203 and the sixth transistor 206, and the seventhtransistor 207 are alternately turned on. Therefore, by inputtingL-level signals whose potentials are changed to the wirings 212 b and212 c and by changing the direction of current flowing through the sixthtransistor 206 and the seventh transistor 207 (switching the sources andthe drains) as described in the above embodiments, concentration of anelectric field on a source side or a drain side is suppressed. Thus,deterioration in the transistors can be suppressed. Further, a structuremay be used in which the direction of current flowing through the thirdtransistor 203 is changed (the source and the drain are switched) byinput of an L-level signal whose potential is changed to the wiring 212a.

Note that at least two of the wirings 212 a, 212 b, and 212 c may beelectrically connected to each other. In the case where the wirings 212b and 212 c are electrically connected to each other, the sixthtransistor 206 and the seventh transistor 207 serve as the transistor112 a and the transistor 112 b in FIGS. 5A to 5C. In the case where thewirings 212 b and 212 c are electrically isolated from each other anddifferent signals are supplied, the sixth transistor 206 and the seventhtransistor 207 serve as the transistor 112 and the transistor 114 inFIGS. 12A to 12C.

Note a circuit structure which can be used for the gate driver is notlimited to the circuit structure in FIG. 23. For example, as illustratedin FIG. 24, a structure may be used in which first to twelfthtransistors 221 to 232 are provided in respective stages (here, n piecesof stages).

In FIG. 24, an output signal from the (n−1)th stage is input to a gateof the first transistor 221. One of a source and a drain of the firsttransistor 221 is electrically connected to a wiring 241. The other ofthe source and the drain of the first transistor 221 is electricallyconnected to a gate of the fourth transistor 224, a gate of the eighthtransistor 228, a gate of the ninth transistor 229, and a gate of theeleventh transistor 231. An output signal from the (n+1)th stage isinput to a gate of the second transistor 222. One of a source and adrain of the second transistor 222 is electrically connected to a wiring242 a. The other of the source and the drain of the second transistor222 is electrically connected to the gate of the fourth transistor 224,the gate of the eighth transistor 228, the gate of the ninth transistor229, and the gate of the eleventh transistor 231. A gate of the thirdtransistor 223 is electrically connected to a drain of the thirdtransistor 223 (the third transistor 223 is diode-connected). A sourceof the third transistor 223 is electrically connected to a gate of thefifth transistor 225. The drain of the third transistor 223 iselectrically connected to the wiring 241. One of a source and a drain ofthe fourth transistor 224 is electrically connected to the wiring 242 a.The other of the source and the drain of the fourth transistor 224 iselectrically connected to the gate of the fifth transistor 225.

One of a source and a drain of the fifth transistor 225 is electricallyconnected to the wiring 241. The other of the source and the drain ofthe fifth transistor 225 is electrically connected to a gate of theseventh transistor 227, a gate of the tenth transistor 230, and a gateof the twelfth transistor 232. The output signal from the (n−1)th stageis input to a gate of the sixth transistor 226. One of a source and adrain of the sixth transistor 226 is electrically connected to thewiring 242 a. The other of the source and the drain of the sixthtransistor 226 is electrically connected to the gate of the seventhtransistor 227, the gate of the tenth transistor 230, and the gate ofthe twelfth transistor 232. One of a source and a drain of the seventhtransistor 227 is electrically connected to the wiring 242 a. The otherof the source and the drain of the seventh transistor 227 iselectrically connected to the gate of the fourth transistor 224, thegate of the eighth transistor 228, the gate of the ninth transistor 229,and the gate of the eleventh transistor 231. One of a source and a drainof the eighth transistor 228 is electrically connected to the wiring 242a. The other of the source and the drain of the eighth transistor 228 iselectrically connected to the gate of the seventh transistor 227, thegate of the tenth transistor 230, and the gate of the twelfth transistor232.

The clock signal (CLK) or the inverted clock signal (CLKB) is input toone of a source and a drain of the ninth transistor 229. One of a sourceand a drain of the tenth transistor 230 is electrically connected to awiring 242 b. The clock signal or the inverted clock signal is input toone of a source and a drain of the eleventh transistor 231. The other ofthe source and the drain of the eleventh transistor 231 is electricallyconnected to a wiring 243. One of a source and a drain of the twelfthtransistor 232 is electrically connected to the wiring 242 b. The otherof the source and the drain of the twelfth transistor 232 iselectrically connected to the wiring 243.

In the structure illustrated in FIG. 24, in a non-selection period, theseventh transistor 227, the tenth transistor 230, and the twelfthtransistor 232 are kept on. Therefore, by inputting L-level signals orL-level signals whose potentials are changed to the wirings 242 a and242 b and by changing the direction of current flowing through theseventh transistor 227, the tenth transistor 230, and the twelfthtransistor 232 (switching the sources and the drains), concentration ofan electric field on a source side or a drain side is suppressed. Thus,deterioration in the transistors can be suppressed.

Note that although n-channel transistors are used as the first totwelfth transistors 221 to 232 in FIG. 24, p-channel transistors may beused. In addition, without provision of the ninth transistor 229 and thetenth transistor 230, a signal which is output from the wiring 243 maybe output to the (n−1)th stage and the (n+1)th stage. Further, thewiring 242 a and the wiring 242 b may be electrically connected to eachother. Furthermore, an H-level signal whose potential is changed may beinput to the wiring 241. In this case, by changing the direction ofcurrent flowing through the fifth transistor 225 (switching the sourceand the drain), concentration of an electric field on a source side or adrain side is suppressed. Thus, deterioration in the transistor can besuppressed. Note that in FIG. 24, signals which are input to the wiringsare not limited to these signals, and signals can be selected asappropriate.

Further, as illustrated in FIG. 25, a structure may be used in whichfirst to fifth transistors 251 to 255 are provided in respective stages(here, n pieces of stages).

In FIG. 25, an inverted clock signal (CLKB) is input to a gate of thefirst transistor 251. An output signal from the (n−1)th stage is inputto one of a source and a drain of the first transistor 251. The other ofthe source and the drain of the first transistor 251 is electricallyconnected to a gate of the fourth transistor 254. A gate of the secondtransistor 252 is electrically connected to a wiring 263. One of asource and a drain of the second transistor 252 is electricallyconnected to a wiring 262 a. The other of the source and the drain ofthe second transistor 252 is electrically connected to a gate of thefifth transistor 255. The inverted clock signal (CLKB) is input to agate of the third transistor 253. One of a source and a drain of thethird transistor 253 is electrically connected to a wiring 264. Theother of the source and the drain of the third transistor 253 iselectrically to the gate of the fifth transistor 255. The gate of thefourth transistor 254 is electrically connected to the other of thesource and the drain of the first transistor 251. A clock signal (CLK)is input to one of a source and a drain of the fourth transistor 254.The other of the source and the drain of the fourth transistor 254 iselectrically connected to the wiring 263. One of a source and a drain ofthe fifth transistor 255 is electrically connected to a wiring 262 b.The other of the source and the drain of the fifth transistor 255 iselectrically connected to the wiring 263.

In the structure illustrated in FIG. 25, in a non-selection period, thefifth transistor 255 is kept on. Therefore, by inputting an H-levelsignal whose potential is changed to the wiring 262 b and by changingthe direction of current flowing through the fifth transistor 255(switching the source and the drain), concentration of an electric fieldon a source side or a drain side is suppressed. Thus, deterioration inthe transistor can be suppressed.

Note that although p-channel transistors are used as the first to fifthtransistors 251 to 255 in FIG. 25, n-channel transistors may be used.Further, the wiring 262 a and the wiring 262 b may be electricallyconnected to each other. Furthermore, in FIG. 25, signals which areinput to the wirings are not limited to these signals, and signals canbe selected as appropriate.

Further, as illustrated in FIG. 26, a structure may be used in whichfirst to tenth transistors 271 to 280 are provided in respective stages(here, n pieces of stages).

In FIG. 26, a first clock signal is input to a gate of the firsttransistor 271. A first input signal is input to one of a source and adrain of the first transistor 271. The other of the source and the drainof the first transistor 271 is electrically connected to a gate of theeighth transistor 278. A second clock signal is input to a gate of thesecond transistor 272. A second input signal is input to one of a sourceand a drain of the second transistor 272. The other of the source andthe drain of the second transistor 272 is electrically connected to thegate of the eighth transistor 278. One of a source and a drain of thethird transistor 273 is electrically connected to a wiring 282 a. Theother of the source and the drain of the third transistor 273 iselectrically connected to the gate of the eighth transistor 278. One ofa source and a drain of the fourth transistor 274 is electricallyconnected to the wiring 282 a. The other of the source and the drain ofthe fourth transistor 274 is electrically connected to one of a sourceand a drain of the fifth transistor 275 and one of a source and a drainof the sixth transistor 276. A third clock signal is input to a gate ofthe fifth transistor 275. The one of the source and the drain of thefifth transistor 275 is electrically connected to the other of thesource and the drain of the fourth transistor 274. The other of thesource and the drain of the fifth transistor 275 is electricallyconnected to the gate of the eighth transistor 278.

A gate of the sixth transistor 276 is electrically connected to a wiring281. The one of the source and the drain of the sixth transistor 276 iselectrically connected to the other of the source and the drain of thefourth transistor 274. The other of the source and the drain of thesixth transistor 276 is electrically connected to a gate of the ninthtransistor 279. A gate of the seventh transistor 277 is electricallyconnected to a drain of the seventh transistor 277 (the seventhtransistor 277 is diode-connected). A source of the seventh transistor277 is electrically connected to the gate of the third transistor 273,the gate of the fourth transistor 274, and a gate of the tenthtransistor 280. The third clock signal is input to the drain of theseventh transistor 277. One of a source and a drain of the eighthtransistor 278 is electrically connected to the wiring 282 a. The otherof the source and the drain of the eighth transistor 278 is electricallyconnected to the gate of the third transistor 273, the gate of thefourth transistor 274, and the gate of the tenth transistor 280. Afourth clock signal is input to one of a source and a drain of the ninthtransistor 279. The other of the source and the drain of the ninthtransistor 279 is electrically connected to a wiring 283. One of asource and a drain of the tenth transistor 280 is electrically connectedto a wiring 282 b. The other of the source and the drain of the tenthtransistor 280 is electrically connected to the wiring 283.

In the structure illustrated in FIG. 26, in a non-selection period, thetenth transistor 280 is kept on. Therefore, by inputting an H-levelsignal whose potential is changed to the wiring 282 b and by changingthe direction of current flowing through the tenth transistor 280(switching the source and the drain), concentration of an electric fieldon a source side or a drain side is suppressed. Thus, deterioration inthe transistor can be suppressed.

Note that although p-channel transistors are used as the first to tenthtransistors 271 to 280 in FIG. 26, n-channel transistors may be used.Further, the wiring 282 a and the wiring 282 b may be electricallyconnected to each other. Furthermore, in FIG. 26, signals which areinput to the wirings are not limited to these signals, and signals canbe selected as appropriate.

Note that although the case where any of the structures described in theabove embodiments is used for a driver circuit such as a gate driver ora source driver is described in this embodiment, this embodiment is notlimited to this. Any of the structures described in the aboveembodiments can also be used in the case where a potential of acapacitor line is controlled or a potential of a common electrode iscontrolled.

Note that the structure described in this embodiment can be combinedwith a different structure described in this specification (including astructure described in any of the other embodiments) as appropriate.

Embodiment 5

In this embodiment, structures of transistors which can be used for thecircuit described in any of the above embodiments are described.Transistors can be broadly classified according to materials used forsemiconductor layers included in the transistors. The materials used forsemiconductor layers can be classified into two categories: a siliconbased material which contains silicon as its main component, and anon-silicon based material which does not contain silicon as its maincomponent. Examples of the silicon based material are amorphous silicon(a-Si:H), microcrystal silicon (μc-Si), polysilicon (p-Si), singlecrystalline silicon (c-Si), and the like. Examples of the non-siliconbased material are compound semiconductors such as gallium arsenide(GaAs), oxide semiconductors such as zinc oxide (ZnO) and an oxidecontaining indium, gallium, and zinc (InGaZnO), and the like.

The use of amorphous silicon or microcrystal silicon for semiconductorlayers of transistors has advantages of high uniformity ofcharacteristics of the transistors and low manufacturing cost, and isparticularly effective in manufacturing transistors over a largesubstrate with a diagonal of more than 500 mm. Examples of a structureof a capacitor and a structure of a transistor in which amorphoussilicon or microcrystal silicon is used for a semiconductor layer aredescribed below.

FIG. 27A illustrates cross-sectional structures of a top-gate transistorand a capacitor.

A first insulating film (an insulating film 5142) is formed over asubstrate 5141. The first insulating film can have a function of a basefilm which prevents impurities from a substrate side from adverselyaffecting a semiconductor layer and changing characteristics of thetransistor. Note that as the first insulating film, a single layer or astacked layer of a silicon oxide film, a silicon nitride film, a siliconoxynitride film (SiO_(x)N_(y)), or the like can be used. In particular,the silicon nitride film is dense and has high barrier properties, sothat the first insulating film preferably contains silicon nitride. Notethat the first insulating film is not necessarily formed. When the firstinsulating film is not formed, reduction in the number of steps andmanufacturing cost and increase in yield can be realized.

A first conductive layer (a conductive layer 5143, a conductive layer5144, and a conductive layer 5145) is formed over the first insulatingfilm. The conductive layer 5143 includes a portion which serves as oneof a source and a drain of a transistor 5158. The conductive layer 5144includes a portion which serves as the other of the source and the drainof the transistor 5158. The conductive layer 5145 includes a portionwhich serves as a first electrode of a capacitor 5159. Note that for thefirst conductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb,Si, Zn, Fe, Ba, Ge, or the like; or an alloy of any of these elementscan be used. Alternatively, a stacked layer of any of these elements(including the alloy thereof) can be used.

A first semiconductor layer (a semiconductor layer 5146 and asemiconductor layer 5147) is formed over the conductive layers 5143 and5144. The semiconductor layer 5146 includes a portion which serves asone of a source and a drain. The semiconductor layer 5147 includes aportion which serves as the other of the source and the drain. Note thatfor the first semiconductor layer, silicon containing phosphorus or thelike can be used, for example.

A second semiconductor layer (a semiconductor layer 5148) is formedbetween the conductive layer 5143 and the conductive layer 5144 and overthe first insulating film. In addition, part of the semiconductor layer5148 extends over the conductive layer 5143 and the conductive layer5144. The semiconductor layer 5148 includes a portion which serves as achannel region of the transistor 5158. Note that as the secondsemiconductor layer, a semiconductor layer having non-crystallinity,such as an amorphous silicon (a-Si:H) layer, or a semiconductor layersuch as a microcrystalline silicon (μc-Si) layer, or the like can beused.

A second insulating film (an insulating film 5149 and an insulating film5150) is formed so as to cover at least the semiconductor layer 5148 andthe conductive layer 5145. The second insulating film serves as a gateinsulating film. Note that as the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, asilicon oxynitride film (SiO_(x)N_(y)), or the like can be used.

Note that as the second insulating film which is in contact with thesecond semiconductor layer, a silicon oxide film is preferably used.This is because trap levels at an interface between the secondsemiconductor layer and the second insulating film is decreased.

Note that in the case where the second insulating film is in contactwith Mo, a silicon oxide film is preferably used as the secondinsulating film which is in contact with Mo. This is because the siliconoxide film does not oxidize Mo.

A second conductive layer (a conductive layer 5151 and a conductivelayer 5152) is formed over the second insulating film. The conductivelayer 5151 includes a portion which serves as a gate electrode of thetransistor 5158. The conductive layer 5152 serves as a second electrodeof the capacitor 5159 or a wiring. Note that for the second conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or the like; or an alloy of any of these elements can be used.Alternatively, a stacked layer including any of these elements(including the alloy thereof) can be used.

Note that in steps after firming the second conductive layer, a varietyof insulating films or a variety of conductive films may be formed.

FIG. 27B illustrates cross-sectional structures of an inverted-staggered(bottom-gate) transistor and a capacitor. In particular, the transistorillustrated in FIG. 27B has a channel-etched structure.

A first insulating film (an insulating film 5162) is formed over asubstrate 5161. The first insulating film can have a function of a basefilm which prevents impurities from a substrate side from adverselyaffecting a semiconductor layer and changing characteristics of thetransistor. Note that as the first insulating film, a single layer or astacked layer of a silicon oxide film, a silicon nitride film, a siliconoxynitride film (SiO_(x)N_(y)), or the like can be used. In particular,the silicon nitride film is dense and has high barrier properties, sothat the first insulating film preferably contains silicon nitride. Notethat the first insulating film is not necessarily formed. When the firstinsulating film is not formed, reduction in the number of steps andmanufacturing cost and increase in yield can be realized.

A first conductive layer (a conductive layer 5163 and a conductive layer5164) is formed over the first insulating film. The conductive layer5163 includes a portion which serves as a gate electrode of a transistor5178. The conductive layer 5164 includes a portion which serves as afirst electrode of a capacitor 5179. Note that for the first conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or the like; or an alloy of any of these elements can be used.Alternatively, a stacked layer including any of these elements(including the alloy thereof) can be used.

A second insulating film (an insulating film 5165) is formed so as tocover at least the first conductive layer. The second insulating filmserves as a gate insulating film. Note that as the second insulatingfilm, a single layer or a stacked layer of a silicon oxide film, asilicon nitride film, a silicon oxynitride film (SiO_(x)N_(y)), or thelike can be used.

Note that as the second insulating film which is in contact with asemiconductor layer, a silicon oxide film is preferably used. This isbecause trap levels at an interface between the semiconductor layer andthe second insulating film is decreased.

Note that in the case where the second insulating film is in contactwith Mo, a silicon oxide film is preferably used as the secondinsulating film which is in contact with Mo. This is because the siliconoxide film does not oxidize Mo.

A first semiconductor layer (a semiconductor layer 5166) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by photolithography, an inkjet method, aprinting method, or the like. In addition, part of the semiconductorlayer 5166 extends to a portion over the second insulating film, whichdoes not overlap with the first conductive layer. The semiconductorlayer 5166 includes a portion which serves as a channel region of thetransistor 5178. Note that as the semiconductor layer 5166, asemiconductor layer having non-crystallinity, such as an amorphoussilicon (a-Si:H) layer, or a semiconductor layer such as amicrocrystalline silicon (μc-Si) layer, or the like can be used.

A second semiconductor layer (a semiconductor layer 5167 and asemiconductor layer 5168) is formed over part of the first semiconductorlayer. The semiconductor layer 5167 includes a portion which serves asone of a source and a drain. The semiconductor layer 5168 includes aportion which serves as the other of the source and the drain. Note thatfor the second semiconductor layer, silicon containing phosphorus or thelike can be used, for example.

A second conductive layer (a conductive layer 5169, a conductive layer5170, and a conductive layer 5171) is formed over the secondsemiconductor layer and the second insulating film. The conductive layer5169 includes a portion which serves as one of a source and a drain ofthe transistor 5178. The conductive layer 5170 includes a portion whichserves as the other of the source and the drain of the transistor 5178.The conductive layer 5171 includes a portion which serves as a secondelectrode of the capacitor 5179. Note that for the second conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge. or the like; or an alloy of any of these elements can be used.Alternatively, a stacked layer including any of these elements(including the alloy thereof) can be used.

Note that in steps after forming the second conductive layer, a varietyof insulating films or a variety of conductive films may be formed.

Note that in steps of manufacturing a channel-etched transistor, thefirst semiconductor layer and the second semiconductor layer can besuccessively formed. Further, the first semiconductor layer and thesecond semiconductor layer can be formed using the same mask.

After the second conductive layer is formed, part of the secondsemiconductor layer can be removed by using the second conductive layeras a mask. Alternatively, by removing part of the second semiconductorlayer by using the same mask used for the second conductive layer, thechannel region of the transistor can be formed. Accordingly, it is notnecessary to use an additional mask which is used only for removing partof the second semiconductor layer. Thus, a manufacturing process can besimplified, so that manufacturing cost can be reduced. Here, part of thefirst semiconductor layer below a region where the second semiconductorlayer is removed serves as the channel region of the transistor.

FIG. 27C illustrates cross-sectional structures of an inverted-staggered(bottom-gate) transistor and a capacitor. In particular, the transistorillustrated in FIG. 27C has a channel protection (etch stop) structure.

A first insulating film (an insulating film 5182) is formed over asubstrate 5181. The first insulating film can have a function of a basefilm which prevents impurities from a substrate side from adverselyaffecting a semiconductor layer and changing characteristics of thetransistor. Note that as the first insulating film, a single layer or astacked layer of a silicon oxide film, a silicon nitride film, a siliconoxynitride film (SiO_(x)N_(y)), or the like can be used. In particular,the silicon nitride film is dense and has high barrier properties, sothat the first insulating film preferably contains silicon nitride. Notethat the first insulating film is not necessarily formed. When the firstinsulating film is not formed, reduction in the number of steps andmanufacturing cost and increase in yield can be realized.

A first conductive layer (a conductive layer 5183 and a conductive layer5184) is formed over the first insulating film. The conductive layer5183 includes a portion which serves as a gate electrode of a transistor5198. The conductive layer 5184 includes a portion which serves as afirst electrode of a capacitor 5199. Note that for the first conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or the like; or an alloy of any of these elements can be used.Alternatively, a stacked layer including any of these elements(including the alloy thereof) can be used.

A second insulating film (an insulating film 5185) is formed so as tocover at least the first conductive layer. The second insulating filmserves as a gate insulating film. Note that as the second insulatingfilm, a single layer or a stacked layer of a silicon oxide film, asilicon nitride film, a silicon oxynitride film (SiO_(x)N_(y)), or thelike can be used.

Note that as the second insulating film which is in contact with asemiconductor layer, a silicon oxide film is preferably used. This isbecause trap levels at an interface between the semiconductor layer andthe second insulating film is decreased.

Note that in the case where the second insulating film is in contactwith MO, a silicon oxide film is preferably used as the secondinsulating film which is in contact with Mo. This is because the siliconoxide film does not oxidize Mo.

A first semiconductor layer (a semiconductor layer 5186) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by photolithography, an inkjet method, aprinting method, or the like. In addition, part of the semiconductorlayer 5186 extends to a portion over the second insulating film, whichdoes not overlap with the first conductive layer. The semiconductorlayer 5186 includes a portion which serves as a channel region of thetransistor 5198. Note that as the semiconductor layer 5186, asemiconductor layer having non-crystallinity, such as an amorphoussilicon (a-Si:H) layer, or a semiconductor layer such as amicrocrystalline silicon (μc-Si) layer, or the like can be used.

A third insulating film (an insulating film 5192) is formed over part ofthe first semiconductor layer. The insulating film 5192 prevents thechannel region of the transistor 5198 from being etched away. That is,the insulating film 5192 serves as a channel protective film (an etchstop film). Note that as the third insulating film, a single layer or astacked layer of a silicon oxide film, a silicon nitride film, a siliconoxynitride film (SiO_(x)N_(y)), or the like can be used.

A second semiconductor layer (a semiconductor layer 5187 and asemiconductor layer 5188) is formed over part of the first semiconductorlayer and part of the third insulating film. The semiconductor layer5187 includes a portion which serves as one of a source and a drain. Thesemiconductor layer 5188 includes a portion which serves as the other ofthe source and the drain. Note that for the second semiconductor layer,silicon containing phosphorus or the like can be used, for example.

A second conductive layer (a conductive layer 5189, a conductive layer5190, and a conductive layer 5191) is formed over the secondsemiconductor layer. The conductive layer 5189 includes a portion whichserves as one of a source and a drain of the transistor 5198. Theconductive layer 5190 includes a portion which serves as the other ofthe source and the drain of the transistor 5198. The conductive layer5191 includes a portion which serves as a second electrode of thecapacitor 5199. Note that for the second conductive layer, Ti, Mo, Ta,Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, Ge, or the like; oran alloy of any of these elements can be used. Alternatively, a stackedlayer including any of these elements (including the alloy thereof) canbe used.

Note that in steps after forming the second conductive layer, a varietyof insulating films or a variety of conductive films may be formed.

The use of polysilicon for semiconductor layers of transistors hasadvantages of high mobility of the transistors and low manufacturingcost. Moreover, since little deterioration in characteristics over timeoccurs, a highly reliable device can be obtained. Examples of astructure of a capacitor and a structure of a transistor in whichpolysilicon is used for a semiconductor layer are described below.

FIG. 27D illustrates cross-sectional structures of a bottom-gatetransistor and a capacitor.

A first insulating film (an insulating film 5202) is formed over asubstrate 5201. The first insulating film can have a function of a basefilm which prevents impurities from a substrate side from adverselyaffecting a semiconductor layer and changing characteristics of thetransistor. Note that as the first insulating film, a single layer or astacked layer of a silicon oxide film, a silicon nitride film, a siliconoxynitride film (SiO_(x)N_(y)), or the like can be used. In particular,the silicon nitride film is dense and has high barrier properties, sothat the first insulating film preferably contains silicon nitride. Notethat the first insulating film is not necessarily formed. When the firstinsulating film is not formed, reduction in the number of steps andmanufacturing cost and increase in yield can be realized.

A first conductive layer (a conductive layer 5203 and a conductive layer5204) is formed over the first insulating film. The conductive layer5203 includes a portion which serves as a gate electrode of a transistor5218. The conductive layer 5204 includes a portion which serves as afirst electrode of a capacitor 5219. Note that for the first conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or the like; or an alloy of any of these elements can be used.Alternatively, a stacked layer including any of these elements(including the alloy thereof) can be used.

A second insulating film (an insulating film 5214) is formed so as tocover at least the first conductive layer. The second insulating filmserves as a gate insulating film. Note that as the second insulatingfilm, a single layer or a stacked layer of a silicon oxide film, asilicon nitride film, a silicon oxynitride film (SiO_(x)N_(y)), or thelike can be used.

Note that as the second insulating film which is in contact with thesemiconductor layer, a silicon oxide film is preferably used. This isbecause trap levels at an interface between the semiconductor layer andthe second insulating film is decreased.

Note that in the case where the second insulating film is in contactwith Mo, a silicon oxide film is preferably used as the secondinsulating film which is in contact with Mo. This is because the siliconoxide film does not oxidize Mo.

A semiconductor layer is formed in part of a portion over the secondinsulating film, which overlaps with the first conductive layer, byphotolithography, an inkjet method, a printing method, or the like. Inaddition, part of the semiconductor layer extends to a portion over thesecond insulating film, which does not overlap with the first conductivelayer. The semiconductor layer includes a channel formation region (achannel formation region 5210), lightly doped drain (LDD) regions (LDDregions 5208 and 5209), and impurity regions (impurity regions 5205,5206, and 5207). The channel formation region 5210 serves as a channelformation region of the transistor 5218. The LDD regions 5208 and 5209serve as LDD regions of the transistor 5218. Note that formation of theLDD regions 5208 and 5209 can prevent application of a high electricfield to a drain of the transistor, so that reliability of thetransistor can be improved. Note that the LDD region is not necessarilyformed. In this case, a manufacturing process can be simplified, so thatmanufacturing cost can be reduced. The impurity region 5205 includes aportion which serves as one of a source and a drain of the transistor5218. The impurity region 5206 includes a portion which serves as theother of the source and the drain of the transistor 5218. The impurityregion 5207 includes a portion which serves as a second electrode of thecapacitor 5219.

A contact hole is selectively formed in part of a third insulating film(an insulating film 5211). The insulating film 5211 serves as aninterlayer film. For the third insulating film, an inorganic material(e.g., silicon oxide, silicon nitride, or silicon oxynitride), anorganic compound material having a low dielectric constant (e.g., aphotosensitive or non-photosensitive organic resin material), or thelike can be used. Alternatively, a material including siloxane can beused. Note that siloxane is a material having a skeleton structure bythe bond of silicon (Si) and oxygen (O). An organic group (e.g., analkyl group or aromatic hydrocarbon) or a fluoro group may be used as asubstituent. A fluoro group may be contained in the organic group.

A second conductive layer (a conductive layer 5212 and a conductivelayer 5213) is formed over the third insulating film. The conductivelayer 5212 is electrically connected to the other of the source and thedrain of the transistor 5218 through the contact hole formed in thethird insulating film. Therefore, the conductive layer 5212 includes aportion which serves as the other of the source or the drain of thetransistor 5218. In the case where the conductive layer 5213 and theconductive layer 5204 are electrically connected to each other in aportion which is not illustrated, the conductive layer 5213 includes aportion which serves as the first electrode of the capacitor 5219.Alternatively, in the case where the conductive layer 5213 iselectrically connected to the impurity region 5207 in a portion which isnot illustrated, the conductive layer 5213 includes the portion whichserves as the second electrode of the capacitor 5219. Alternatively, inthe case where the conductive layer 5213 is not electrically connectedto the conductive layer 5204 and the impurity region 5207, a capacitorwhich is different from the capacitor 5219 is formed. In this capacitor,the conductive layer 5213, the impurity region 5207, and the insulatingfilm 5211 are used as a first electrode, a second electrode, and aninsulating film, respectively. Note that for the second conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba,Ge, or the like; or an alloy of any of these elements can be used.Alternatively, a stacked layer including any of these elements(including the alloy thereof) can be used.

Note that in steps after forming the second conductive layer, a varietyof insulating films or a variety of conductive films may be formed.

Note that the transistor in which polysilicon is used for asemiconductor layer can have a top-gate structure.

Note that the structure described in this embodiment can be combinedwith a different structure described in this specification (including astructure described in any of the other embodiments) as appropriate.

Embodiment 6

In this embodiment, a variety of electronic devices including thesemiconductor device described in any of the above embodiments aredescribed with reference to drawings.

Examples of electronic devices including the semiconductor devicedescribed in any of the above embodiments are television sets, camerassuch as video cameras and digital cameras, goggle-type displays(head-mounted displays), navigation systems, audio reproducing devices(e.g., car audio equipment or audio component sets), laptops, gamemachines, portable information terminals (e.g., mobile computers, mobilephones, portable game machines, or e-book readers), image reproducingdevices provided with recording media (specifically devices whichreproduce the content of recording media such as DVDs (digital versatiledisc) and have displays for displaying the reproduced images), and thelike. Specific examples of such electronic devices are described inFIGS. 28A to 28E.

FIG. 28A illustrates a display device, which includes a housing 8001, asupport base 8002, a display portion 8003, speaker portions 8004, videoinput terminals 8005, and the like. The display device is manufacturedby using the structure described in any of the above embodiments for thedisplay portion 8003. Note that the display device includes in itscategory all devices for displaying information, for example, devicesfor a personal computer, for receiving IV broadcasting, and fordisplaying an advertisement. By using the display device described inany of the above embodiments, a display device having high reliabilitycan be provided.

FIG. 28B illustrates a computer, which includes a main body 8101, ahousing 8102, a display portion 8103, a keyboard 8104, an externalconnection port 8105, a pointing device 8106, and the like. Note thatthe computer is manufactured by using the structure described in any ofthe above embodiments for the display portion 8103. By using the displaydevice described in any of the above embodiments, a computer having highreliability can be provided.

FIG. 28C illustrates a video camera, which includes a display portion8202, an external connection port 8204, a remote control receivingportion 8205, an image receiving portion 8206, operation keys 8209, andthe like. Note that the video camera is manufactured by using thestructure described in any of the above embodiments for the displayportion 8202. By using the display device described in any of the aboveembodiments, a video camera having high reliability can be provided.

FIG. 28D illustrates a mobile phone, which includes a display portion8403, an audio input portion 8404, an audio output portion 8405,operation keys 8406, an external connection port 8407, and the like.Note that the mobile phone is manufactured by using the structuredescribed in any of the above embodiments for the display portion 8403.Further, the mobile phone may have an infrared communication function, atelevision receiving function, or the like. By using the display devicedescribed in any of the above embodiments, a mobile phone having highreliability can be provided.

FIG. 28E illustrates a table lamp, which includes a lighting portion8301, a shade 8302, an adjustable arm 8303, a switch 8305, and the like.Note that the table lamp is manufactured by using the structuredescribed in any of the above embodiments for the lighting portion 8301.Note that the lamp includes ceiling lights, wall lights, and the like inits category. By using the display device described in any of the aboveembodiments, a table lamp having high reliability can be provided.

As described above, electronic devices and lamps can be obtained byusing the display device described in any of the above embodiments. Theapplication range of the display device described in any of the aboveembodiments is extremely wide and the display device described in any ofthe above embodiments can be used for electronic devices in all fields.

Note that the structure described in this embodiment can be combinedwith a different structure described in this specification (including astructure described in any of the other embodiments) as appropriate.

This application is based on Japanese Patent Application serial no.2008-292197 filed with Japan Patent Office on Nov. 14, 2008, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device, comprising: a firsttransistor, a second transistor, a third transistor, and a fourthtransistor wherein one of a source and a drain of the first transistoris directly connected to a first wiring, wherein the other of the sourceand the drain of the first transistor is directly connected to a gateline, wherein one of a source and a drain of the second transistor isdirectly connected to a second wiring, wherein the other of the sourceand the drain of the second transistor is directly connected to the gateline, wherein one of a source and a drain of the fourth is directlyconnected to the second wiring, wherein the other of the source and thedrain of the fourth transistor is directly connected to the gate line,wherein by turning on the first transistor, a signal that turns on thethird transistor is supplied to the gate line, wherein by turning on thesecond transistor, a signal that turns off the third transistor issupplied to the gate line, wherein a gate of the third transistor isdirectly connected to the gate line, wherein by turning on the fourthtransistor, a signal that turns off the third transistor is supplied tothe gate line, and wherein a signal whose potential is changed between afirst potential and a second potential is supplied to the second wiring.2. A semiconductor device, comprising: a gate driver, a first pixel, anda second pixel, wherein the gate driver comprises a first transistor, asecond transistor, a third transistor, and a fourth transistor, whereinthe first pixel comprises a fifth transistor, wherein the second pixelcomprises a sixth transistor, wherein one of a source and a drain of thefirst transistor is directly connected to a first wiring, wherein theother of the source and the drain of the first transistor is directlyconnected to a first gate line, wherein one of a source and a drain ofthe second transistor is directly connected to a second wiring, whereinthe other of the source and the drain of the second transistor isdirectly connected to the first gate line, wherein by turning on thefirst transistor, a signal that turns on the fifth transistor issupplied to the first gate line, wherein by turning on the secondtransistor, a signal that turns off the fifth transistor is supplied tothe first gate line, wherein one of a source and a drain of the thirdtransistor is directly connected to a third wiring, wherein the other ofthe source and the drain of the third transistor is directly connectedto a second gate line, wherein one of a source and a drain of the fourthis directly connected to the second wiring, wherein the other of thesource and the drain of the fourth transistor is directly connected tothe second gate line, wherein by turning on the third transistor, asignal that turns on the sixth transistor is supplied to the second gateline, wherein by turning on the fourth transistor, a signal that turnsoff the sixth transistor is supplied to the second gate line, wherein agate of the fifth transistor is directly connected to the first gateline, wherein a gate of the sixth transistor is directly connected tothe second gate line, wherein a clock signal is supplied to the firstwiring, and wherein a signal whose potential is changed between a firstpotential and a second potential is supplied to the second wiring.
 3. Asemiconductor device, comprising: a first stage; a second stage thatcomprises a first transistor, a second transistor, a third transistor, afourth transistor, a fifth transistor, a sixth transistor, a seventhtransistor, an eighth transistor, a ninth transistor, a tenthtransistor, an eleventh transistor, and a twelfth transistor; and athird stage, wherein one of a source and a drain of the firsttransistor, one of a source and a drain of the second transistor, a gateof the fourth transistor, one of a source and a drain of the seventhtransistor, a gate of the eighth transistor, a gate of the ninthtransistor, and a gate of the eleventh transistor are directly connectedto each other, wherein a gate of the first transistor, and a gate of thesixth transistor are directly connected to the first stage, wherein agate of the second transistor is directly connected to the third stagewherein one of a source and a drain of the third transistor, and a gateof the third transistor are directly connected to each other, whereinthe other of the source and the drain of the third transistor, one of asource and a drain of the fourth transistor, and a gate of the fifthtransistor are directly connected to each other, wherein one of a sourceand a drain of the fifth transistor, one of a source and a drain of thesixth transistor, a gate of the seventh transistor, one of a source anda drain of the eighth transistor, a gate of the tenth transistor, and agate of the twelfth transistor are directly connected to each other,wherein one of a source and a drain of the ninth transistor, and one ofa source and a drain of the tenth transistor are directly connected tothe first stage and the third stage, wherein one of a source and a drainof the eleventh transistor, and one of a source and a drain of thetwelfth transistor are directly connected to a pixel, wherein the otherof the source and the drain of the ninth transistor, and the other ofthe source and the drain of the eleventh transistor are directlyconnected to a first wiring, wherein the other of the source and thedrain of the second transistor is directly connected to a second wiring,and wherein the other of the source and the drain of the twelfthtransistor is directly connected to a third wiring.